Multiclassing and spell progression is weird, see: levels in the new class stack with the old for spell progression for old spell in such ways as highest green power level, at least when switching to a natural caster. Also, there is a caster level penalty of four for the six gishes, and one for the four non-casters, effective immediately. This applies to all schools! i.e. a level eleven bishop, level five priest has an effective caster level of sixteen, for, all, books. (Except for learning spells, of course.) Bishops are absurd. I hope I didn't butcher that.
Dual wielding is worse than most styles, until past the point the game normally ends for most classes in a party of six. One exception might be a ninja, actually, where mauler/nunchaka might overtake martial arts before the game ends, but then there's just sticking him with a dread spear... The problem is that the accuracy/attacks & swing penalty is a percentage of close combat/weapon skill(s).
Sure, maces are great for priests and bishops, just like giving an alchemist the guaranteed shillelah early is decent. But slings get nine percent k.o. and then nineteen when maces finally give more... but do less damage, even with the penalty to ranged damage bonuses. Shields are good for their a.c. and maybe stat bonus. The resistances and skill bonuses aren't all that relevant. Whips are kinda awful otherwise, since they have the same penalties as ranged attacks.
The problem is that, for most of the game, damage is arguably more important than status effects. Of course, weight, skill increases, attribute bonuses, to-hit, and initiative bonuses are also important. Once effects go over fifteen percent, then they start mattering a lot more. Also, stacking stat effects, like staff of doom, Zatoichi bo, or mindblast. But it's still more gravy than meat, if you will. That's why the omnigun is rather terrible: all statuses, no damage. Disruptor is similar, but at least it's early, crock early—Which, for me, is around level eleven.
P.S. why would you ever rely on items for long-terms buffs outside of challenges? Unless it's to grind music/engineering via quick save/load abuse. Which I guess is nice if you like backfires. Or artifacts, I guess, but then there are scrolls of identify item… (Well, detect secrets, true, true.)
dtgreene: By the way, for Giant's Sword users, what build do you prefer?
Fighter, samurai, or ranger, in that order. But, usually, I just give my Mook rangers polearms, make my fighters dracon, and I'd probably prefer shields on my samurai. I'd only use a Mook lord or valkyrie in a four giants playthrough, for divinity.