Part of the problem is that Fighter/Mage - without any kits attached - is already one of the most powerful class combos in the game. Arguably the most powerful at higher levels. Kensai is basically just all gravy on top of this, provided you're using an offensive melee combat style. For the most part, a mage can replicate all those immunities quite easily. Undispellable is actually a relatively minor issue, and so long as you're careful not to accidentally dispel your own enchantments it rarely comes up. When it does, there is an obvious recourse: simply re-cast your protective magic. Helmets are a non-issue once you get your first ioun stone. Because you use magic as your primary defense, shields are irrelevant. So, you're basically only trading off the ability to use ranged weapons and gauntlets for a rather substantial damage and accuracy increase. It's a very good trade if it matches your combat style.

I read something about a Berserker-Cleric being a powerful combination because of synergy. The Berserker can wear heavy armor and a helmet, and a Cleric can cast in heavy armor and a helmet. The Berserker gets immunity to the frustration spells and the cleric adds undead turning and cleric spells. I do not remember where I read this article.

About a year ago, I intended to give it a try.

If you play with no mods, sure, undispellability is nothing to write home about. In a SCS game, it's another story. You come to treasure and adore anything immune to dispel.
And no, a mage can't replicate chaotic commands or negative plane protection.
Also you form an argument in theoretical void. If protections are gone->recast.
That doesn't work at all in practice. First of all you don't have infinite spell slots. Then there is the issue that multiple layers of protection need equally many rounds to get re-cast. While the enemy pummels you with their attacks. And you would be better off casting something offensive instead.

Helmets are vastly superior to ion stones and shields can replicate permanent and undispellable spell effects, freeing a precious spell slot for something else. Gauntlets can take the place of armor or boost stats or increase offensive potential. Suddenly the rather substantial damage & accuracy bonus doesn't seem so substantial, eh? :)

Keep in mind that I'm not in any way trying to downplay the strength of the kensai->mage dual. It is a very strong combo (and very cool in my view), mainly because, as you already said, fighter/mage combo is very very powerful. I just can't understand why kensai/mage is often called a cheesy class, while there are other stronger choices. If powerplaying is the only concern a zerk/mage is stronger in every single way to the kansai/mage.

@KHHill91
Berserk/cleric is the best F/C combo, kensai/cleric doesn't really make sense and generic-fighter/cleric hasn't got any advantage over the kit.
But, if you don't mind exploiting an engine oversight, ranger/cleric is the best cleric multi/dual you can make, since you get both druid and cleric spellbooks. Ranger is almost as good in combat as a fighter, plus you have ironskin and killer summons too! Win-win... or cheese-cheat... depending on your views :)

We are talking about the base game with minimal mods (see the earlier posts discussing the mods in use by this player) Negative Plane Protection isn't strictly necessary since there's an amulet (given rather early in BG2) that provides exactly that benefit. Because this amulet can only be used by a mage, it's virtually always going to be used by the Fighter/Mage anyways. Chaotic Commands I will grant you, but each of the immunities can be replicated individually by either the right spell or the right piece of equipment. I'd actually agree with you for a pen and paper game (at least until very high levels), but this is a CRPG. Because the game is so lenient on resting, a mage's limited spell slots are only as big an issue as you want them to be. We really are talking about "hypothetical" clairvoyant mage who can predict every upcoming challenge and distribute his spell slots accordingly, and can decide when and how to engage them on his own initiative. Even if you do self-impose rest restrictions (I do), you already know the series from having played it before. The only people for whom the Baldur's Gate series remains something new to explore are likely those without the know-how to fully-utilize the fighter/mage anyways. You don't need that many defenses, and if you do they should probably be in contingencies. Ioun Stones offer different bonuses to the helmets. If you prefer the benefits of the helmets that's fine, but there are plenty of great ioun stones so you're not losing out on an equipment slot, just some selection. There aren't that many shields that offer much in the of replicated spell effects. The one that blocks domination is the only one that comes to mind. I'd say the same thing about the Berserker/Mage. I agree that it's a kick-ass combo (all F/M combos are) but I'm of the opinion that the Kensai is the better of the two.