As long as you adequately gear and build your characters, taking an additional attack or DoT occasionally should not pose a significant issue.
The undead ability to Play Dead works as a functional instant-flee or aggro-shed, which can be quite useful and easily compensates for any combat flaws the undead suffer.
Keep in mind that undead heal with poison and non-'Holy'/hydro aligned healing (so effects like life drain and necromancy skills still restore vitality), but if you are standing in poison it can still be cursed and exploded.
