Actually I think if they describe him as totally devoid of all human emotions, then they really did need that extra fact. The information provided in the villains text allows DMs to make calls on what can and can't work.
However, to make things easier, when designing a villain like the one I was describing above, I guess you can just add the Ex ability (immune to all emotional effects) or some such thing. Perfectly fine for DM call really.
Not even all that powerful really. There's so many more ways to kill a baddy.
I think it's fine for a GM to say "This villain is incapable of human emotions, and is immune to all emotion-based effects as a result." Or really, make him/her/it immune to anything the GM wants.
A PC on the other hand? No, I'd definitely draw the line there. If a PC wants to be immune to emotion effects, there's always the android race.