Although I've heard that many good painters(talking about Golden Demon etc) use white undercoats, I agree that most white undercoated models of mine look absolutely crappy!
But I've found out that you can make a paintjob which looks ok out of it, if you ink or paint the deep spots(those, which you whould keep black on a black undercoated model) with brown.
You still have a bright colour to paint yellow or red on it, but you also have some shadows where you need them.
Let me hear what you guys think about it
I think I would undercoat elves in white, but Empire models definitly not, the newer ones have a quite dark/morbid air, and even the old, "Landsknecht"-models can be painted great on a black undercoat.