Wraiths are hard to use effectively, but could be devastating under the right circumstances. Ghosts and Floating Heads are easier to use than Wraiths, and do similar things - Heads are frighteningly effective at stopping opponents in their tracks. Dark Suckers pull the teeth of defensive units, but you have to protect them. Death Hounds are torpedoes, cheaper than Dragonlings but non-flying. Corpse things are bait for attacks - very hard to use effectively on offense.
Ghouls are all but useless.
Necromancer Farrenghast is still the only evil level 3 spellcaster. The biggest problem I have with Undead is figuring out which units NOT to take. After rejecting the Ghouls, I have to decide my basic tactics before selecting the rest of the units. Vamps, Lampeyes and Dark Suckers are expensive, but it hurts not to take them.