Quote Originally Posted by etrpgb View Post
They come up with the ridiculous "unholy toughness'' to compensate. The real problem is that undead get d12 what ever base dice they had. So a wizard probably enjoys it; a fighter, a barbarian or a dragon...not much.

Maybe it is better just to increase the HP dice? For example... d4 becomes d8, d6 becomes d12, d8 becomes 2d8, d10 becomes... d20, d12 become 2d12.
Average rolls for 2d10 and 1d20 are very different. 11 vs 10.5. So if you're going to double it, it would work better to be consistent and make it 2d4, 2d6, 2d8, 2d10, 2d12. On the other hand, lots of things are dependent on Hit Dice, so doubling their HD would break things. And I doubt many people have d16s lying around, so doubling the number doesn't work too well either.
I think that making their HD become d12s and giving them Unholy Toughness is fair.