Wow. I thought our party had it bad when the GM put us against an I7FV and a Frenzied Berserker.
Basically, the reason why the I7FV is okay for players is because of the entry requirements -- Spell Focus (Abjuration) and Greater Spell Focus (Abjuration) are completely useless feats, and I can only think of maybe two spells that they would apply to. And on top of that, the Veils are an n/day ability. This more or less balances out the power of the I7FV -- but only over time. For NPCs, since you only fight them for a battle or maybe two, the drawbacks of the class are minimized to, essentially, nothing, and the I7FV ends up being very powerful, because the drawbacks don't really matter to them and they can blow all of their Veil uses on just that battle.