A lot of people say that Psionics are useless, or at least, not optimal, compared to other Specialist Casters (Priest, Alchemist, Mage). This is subjective, however.
First, we must notice his/her inherent class bonuses. He/she gets immunity to all mental effects (fear, insanity, turncoat). Nobody else gets all of these (The Samurai is fearless, while the Monk is immune to blind).
The Bard gets to mimic some of these effects (Haste, Insanity, Slow, Cerebral Hemorrhage). The Gadgeteer as well (Hypnotic Lure). However, these are subject to the RNG of the game, and can be cast at any power level the game feels like. Also, some fire spells can't be cast underwater (HASTE?)
I don't understand those that base their characters around the Rousing Drums (Haste instrument). It doesn't work underwater, and there is no guarantee that it will "cast" at power level 7 (sometimes instruments/gadgets cast at power level 1, even when the user is an expert).
The Psionic, more than any other specialist caster, is lopsided in their spells in each realm.
Mental: No discussion here. All eggs are placed in one basket.
Fire: Psionic Fire and Haste are very useful, especially early. Hypnotic Lure can be useful, but I find the sound annoying, and sometimes in rare occasions, the enemy may run into obstacles off limits, preventing you from getting a clean kill. I hate the "prismic" spells, as they can cause any number of conditions, including fear and blinding, which may cause the enemy to run away. This is why I also dislike the Omnigun. A Magic Damage Party likes their targets lined up nicely, not scattered.
Water: Cure Lesser Condition is OK, but the Priest and Alchemist do the same thing. Paralyze is decent if you are facing one tough enemy, but its range is thrown, so you would be better with the Mage with Freeze Flesh and Freeze All. Slow is situational, it has long range, and can slow down a group of tough melee attackers closing in on you, even though the slower animation is annoying.
Air: Every caster has Set/Return to Portal, so no comparison there. He/she can cast Silence, just like the Priest. Shrill Sound is only good early on, just like the Piercing Pipes Bard instrument which does the same thing. Pandemonium I find especially annoying, I don't want my enemies running away in fear.
Earth: Armormelt is decent, as it always works, and reduces the AC of your enemies. I never use Chameleon, as I am trying to earn skill points fighting enemies, but some might find it useful to avoid fights if the enemies aren't too close.
Divine: Heal Wounds is good early on, and even midgame when allies get injured seriously. Instant Death can score a quick kill from long range, but it's hit or miss, and only hits one target. Might to Magic is probably one of the most interesting spells in the game, I use it as often as I can. It actually saps HPs from a monster, and transfers the stolen HPs into SPs, giving the caster in most need of SPs a refill. However, it only works from thrown range, so it is situational. Eye for an Eye I have no use for yet, as early on, enemies don't attack much with magic, thankfully.
I have managed to keep my MDP Psionic balanced, with 100 Psionics skill (125 with the bonus), 95+ skills to the magic realms, and 80+ to Ranged Combat/Throwing and Sling skills. Speed is maxed for Snakespeed (currently in the 20s), Intelligence is at 95 waiting for the +5 in Trynton for Powercast, and Senses and Piety are in the 70s, soon to be maxed by level 27 (I'm level 18 now). As a Human, he has his "trash" stats at 45 (Strength, Dexterity, Vitality).
Any comments?

Looking at the spell list at
https://www.crimsontear.com/gaming/wizardry-6/spells

Here's what the situation looks like for a Psionic by realm:

Fire: No spells, so you have one fewer realm than other casters.
Water: Stamina can have its uses, and I see Haste on this spell list. (No idea how useful that spell actually is in practice.)
Air: Silence, and that's it. If you want decent Air SP, you'll need to pick this spell early, but it's not as if you really need that many SP for just one spell.
Earth: Knock-Knock, Blades, and Armormelt. Only 3 spells, but they're at least potentially useful.
Mental: All of them. Will be the realm that's used most of the time, except when dealing with enemies that have high resistance to mind-affecting spells, which apparently is a significant issue in Wizardry 6 and 7.
Magic: Your choice for healing. Without some of the key Magic spells, you can use these SP for your healing needs. There's not much in terms of healing spells; the only alternative to Heal Wounds is Lifesteal, which happens to be available to the Psionic, but can only heal the user and only if you can find an enemy that it works well on.

The only Psionic only spells are Mental Attack, Minderead, Psionic Blast, Illusion (which is just the Psionic's summon), and Mind Flay (which apparently is broken in Wizardry 6 just like the other spells that hit all enemies, and even if it did work would not be as strong as the mage's Nuclear Blast). So, the question is, "do you really need these spells?", and for Wizardry 6 I have a feeling the answer may be "no".

This list is for Wizardry 6. I believe Wizardry 7 adds some of the other spells you're used to seeing, like Psionic Fire (so the Fire realm now actually has a use) and Shrill Sound (so it's not just Silence for the Air realm).

(You fail to specify which Wizardry game you are talking about.)

I'm talking about Wizardry 8. This is the only one that I'm familiar with, as I started playing it when it came out. W6 and 7 seem like they would be more difficult to get into, although I could be wrong. I did grow up in the 80s and 90s with the NES, so "bad graphics" are not a problem.

thats fun because Pandemonium is my goto spell... it doesn't need a target and covers every enermy in the area i.e, you are blind or they are blocked = no problem

The problem I have with Pandemonium (and similar spells like Terror and Blinding Flash) is that it makes enemies run away, which has the result of prolonging the battle. Given that battles tend to be too long to begin with, and you can't rest or save during combat, this can be a problem, and can result in the player not being able to save before they get unlucky or have to do something outside of the game.

Freeze All has the same targeting as Pandemonium and it doesn't have the same issues, so I prefer that spell instead.

(Incidentally, Slow also has the problem of prolonging combat, albeit for an entirely different reason.)

By the way, a Bishop who focuses on Alchemy and Psionics could be an interesting build, as Psionics is strongest in the realm that Alchemy is weakest (or, more precisely, non-existent). (Worth noting that this is only possible in Wizardry 8; in 6 and 7, Bishops only got Priest and Wizard spells, and were a mess for a couple other reasons.)

I must admit that I the Psionic is my favorite character, the reason is because they seem to be weaker than other classes and I'm a sucker for an underdog. But in my opinion they are only slightly weaker than the other straight caster classes and the resistances are actually really good.

imo the Psionic class is less about damage and more about crowd control so its a different playstyle to what some people like... each to their own of course

Reminds me of how, in Pathfinder: Wrath of the Righteous, I'm playing a Witch (aiming for Trickster), even though that would be one of the "weaker" set-ups.

Then again, in Wizardry 8 my favorite class is Bishop, because I like versatile characters over powerful ones. Failing that, my preference is to favor healing (even in games where it's a sub-optimal strategy), but Priest's aren't that much better than Bishops, and Bards are actually better at out-of-combat healing upon reaching level 11, provided you have actually obtained one copy of a certain instrument.

(This also explains why my endgame Final Fantasy 3 remake party had both a Devout (powerful healer) and a Red Mage (can fight reasonably well in the endgame, but also gets a decent healing spell). Then again, I also had a Bard in the party, for free healing as well as enough damage reduction to make boss fights considerably safer.)

(Worth noting that versatile class choices have to at least be viable; Bishops from earlier Wizardry games aren't, as they learn new spell levels way too slowly.) Then again, this happens to be a game where crowd control is a viable strategy; enemies aren't too resistant until later, and battles last long enough for it to matter. (Contrast this to many JRPGs (and probably some WRPGs) where random battles end too soon for such spells to be useful and bosses are outright immune.) And if your Priest and Alchemist are nauseated or blind, but you Psionic has managed to avoid these effects?

(It's worth having Cure Lesser Condition on everyone who can learn it, though I wish there were a multi-target version of this spell, like there is in some of the Japanese Wizardries.)

I really like the Psionic Blast spell. It's level 5, meaning it can be learned at level 11 for a pure caster. I'm level 18, so I can cast it at max power level 7. It hit from long range, and it targets a whole group of enemies, even if you can only see one of them. This gets rid of one of my biggest annoyances: Fighting a large group of enemies when you can only see a few of them. It actually helped me get rid of a group of 4 level 14 Flesheater Slimes (nasty, as they can disease and drain you from a distance, and get multiple attacks, and have high HPs). I could only see one slime, and its buddies were far away, thankfully, because they were in a small hallway where only one could get to me at a time because they are so big. I repeatedly cast Psionic Blast, and the far away slimes were taking heavy damage, while not being able to hit me. The first one in thrown range went down pretty quickly, then by the time the other 3 went all out and tried to surround me (I was flanked on both sides), the final round of PL7 Fireball and Firebomb got them all. Also, it can make enemies insane, though I wouldn't rely on it.
I also like Insanity. Seeing enemies beating on each other, instead of me, is very enjoyable. However, it doesn't do any damage, and doesn't always work. Also, the Bard can cast it for free (only costs Stamina), once you reach the graveyard on AT Road.
Unfortunately, the Cure Lesser Condition spellbook can't be bought until Sadok, (why doesn't Lord Braffit sell it?), but there is a free hidden one in Arnika, so I will give that to my Psionic. I think it's enough that the Priest and Alchemist picked it at level up, so I don't have to carry around so many potions. This is also why I picked Cure Poison for my Priest and Alchemist (I got tired of carrying around half a million Cure Poison potions).

He really should sell that spell book, along with Holy Water (especially since he has a chance of dropping the Holy Water book if you, for whatever reason, decide to kill him).

(Then again, Lord Braffit is actually a monk (or maybe a psionic), not a priest, so he can't actually cast Holy Water (does the AI ever use that spell?).)

I actually really like your idea about 2 two-school Bishops. Back in the day, my go-to party was Fighter, Rogue, Ranger, Bard, Gadgeteer, Bishop. Gotta find all the hidden items with the Ranger, and 4 easy leveling characters, and one with all magic. The one 4-school Bishop was useless until about Arnika, where I could finally buy some spellbooks, then make a mad dash to Fuzzfas, Crock, Sadok, then Kunar. Only then was she awesome, with all the low level spells bought except Mind Stab, which had to be picked at level up, and most level 6 and 7 spells picked at level up except Banish, which can be bought or found.
She was very powerful, but as the only main caster (the Ranger gets magic more slowly), she struggled to choose between Element Shield and Soul Shield. Usually, most enemies either attack with elemental magic, or Mental/Divine magic, so it's usually an easy choice, but some enemies can attack with any magic, forcing me to guess which one to do first. Also, with the way magic damage in this game works, one caster isn't usually going to wipe the battlefield in one round.
I find the concept of 2 two-school Bishops very fascinating, but I keep getting struck with choice-paralysis. There are six ways to make a two-school Bishop. Priest/Alchemist, Priest/Psionic, Priest/Mage, Alchemist/Psionic, Alchemist/Mage, Psionic/Mage. Focusing on only two schools would mean you would need 14 spell picks for level 6 and 7 spells, if there was no overlap with spellbooks. I assume that, played this way, you wouldn't need to save all spell picks until Arnika, and be dead weight for the rest of the party to carry around.
My personality flaw is to make "perfect" characters, so I would struggle not to attempt to grind up the remaining 2 spellbook skills and try to get all 101 spells for both Bishops. I could see myself struggling with that. This is why I have an MDP so I don't have to concern myself with saving spell picks, and having 4 fast casters means potentially a battlefield wipe in one round.

You could take the approach of starting out as a 2-school Bishop, but then branching out into other schools.

Also, you don't need to master every school you learn spells in. For example, at 15 Wizardry you can learn Enchanted Blade and Missile Shield, 2 of the few mage spells that would be considered vital (and which aren't enemy targeted). A Gadgeteer can cover X-Ray, and every other important mage spell appears in one of the other spellbooks, so you don't *need* any more skill here.

Similarly, I would say that higher level Psionic spells aren't that important; Psionic Blast is nice and unique, but you don't really need any of the 6th and 7th level spells, so having Psionics in the 50s (and decent Mental Magic, of course) is enough to get every important spell in the school.

This means that Priest and Alchemist are the most important spellbooks at higher levels, and even then there's only two Priest spell above 5th level that I consider vital (and one of them is also an Alchemist spell). In fact, you probably only need to get Divinity into the 80s, as your Divine Magic skill should get very high by playing normally, and Falling Stars is pointless if your Alchemy is high enough to learn Earthquake.

I actually enjoy "grinding" the remaining spellbook skills up at least to reasonable levels.

For the saving spell picks issue, I use the same house rule I mentioned earlier:
* Spell picks earned before level 14 can't be used for 6th level spells. (Making an exception for Banish might be reasonable simply because you can acquire that spellbook normally, but I don't think it's worth picking at this point.)
* Spell picks earned before level 18 can't be used for 7th level spells.

With these rules (could be extended to lower levels, but I don't think that's entirely necessary), there's no longer pressure to avoid picking spells, and make the earlier game less fun, just to get more high level spells. This way, I actually have to *choose* which spells I want, rather than just getting them all. And not all the higher level spells are that great anyway; in fact, even just 4 6th level picks (before 7th level spells start becoming available) feels like more than I need; after Quicksand and Resurrection, what else is there at that level?

There's crowd control in other spellbooks.
* Wizardry has Freeze Flesh and Freeze All, which are quite good against Rapax in particular. Also gets Noxious Fumes.
* Divinity has Web, though it seems to lack higher level spells (though at least it gets a multi-target instant death spell at 7th).
* Alchemy gets Noxious Fumes and Toxic Cloud, as well as Blinding Flash. (I *really* like Toxic Cloud, at least when I'm the one using it.)
* Psionics gets Insanity, Psionic Blast, Pandemonium, and Prismic Ray/Chaos.