If it has to be balanced for all characters, then why are fighters easy?
But I know what you mean.
After thinking some more about it, the bane of all spellcasters are mobs and hardhitting monsters with lots of hit points that can't be held or controlled. Neverwinter Nights has those in spades.
But the bane of fighters, the lich/dragon that casts flame shield, improved invisibility, blur, mirror image, stoneskin, protection from magical weapons and/or improved mantle then lets go with the artillery.... NWN is sadly lacking in those.
I don't recall a single spellcaster using any of the above mentioned spells, other than simple invisibility. Or if they did, I didn't notice. In BGII, you noticed. You couldn't take on a lich or dragon or a party with a decent druid or mage and not notice that there were some REALLY annoying spells.
But again, I think you're right about the balancing issue. A mage against a mob of fire giants? Very hard, but very doable. A pure fighter against a lich? No chance, unless you had a wand of pierce magic and dispel magic. And those aren't lying around everywhere.
A serious problem with the single henchman over party style of play, imo. I'm not a D&D'er, but those of my friends who are have told me often that it's all about the well-balanced party, the complementary characters.
Not in NWN.