While a UMD-using warlock can be pretty devastating in PnP, I have yet to see UMD play anything more than a marginal role in my NWN2 game.
I have it maxed out, of course. I believe my warlock's total UMD bonus is +16 or something. However (A) I haven't found any really good wands yet (B) I don't have enough cash to craft even a 1st level wand. I have no idea whether this will change after the area I am currently playing (and got bored with).
Oh, and my UMD performance on scrolls is lackluster. I fail roughly 50% of the time.
So, all in, as I doubt that the majority of NWN2 gameplay will happen at uber-high levels (and, hence, uber-high UMD bonus), I don't think it's unbalanced. I could be wrong, of course.
BTW, I simply don't see how a warlock is more boring to play than a fighter. Compared to true spellcasters, sure. But fighters have done exactly the same thing since the dawn of D&D, and people still play them. Wash, rinse, repeat.