As a more concrete example, take a Bard 6 or Dread Necromancer 6 (Both Tier 3 classes) against a Bloodhulk 6. Which is stronger and more capable? In combat, it's going to depend a fair bit on precise optimization, but out of combat, both the Bard and DN are far more useful than the Bloodhulk, between access to spells and skills. The Bloodhulk, well, he's really good at lifting heavy objects, so if you're building a fortress he's very useful. But that's rather specific. Giving him a few skills means he can do a couple of things out of combat, or at least not be at perpetual risk of ambush.
Bluntly, the biggest drawback for being undead is not having spells. And Undead only have +1/2 BAB, so they aren't brilliant at hitting lots in combat either (excluding magic items, etc.). A good benchmark of melee combat effectiveness is the Warblade or Crusader, so unless the undead creature is running roughly even with them, they are probably in need of a boost.