Grrr, I had this whole lecture on the Undead heroes ready to post, but then my post was rejected for too many smilies, so I press the Back button and BAM, it's all gone. Since my fingers hurt, I don't feel like typing up the entire thing all over again, so I'll just explain it in a nutshell.
The Lich is a bad creeper. Nova is to be used in combat, Armor isn't good enough, Dark Ritual just kills your Ghouls, which is the last thing you want. You want your units ALIVE, not dead, when creeping. He can't tank or create tanks either. Requires lots of Cannibalizing and Ghoul rotation, both of which are too time-consuming to use often. Not good.
The DL is ok, just outclassed by the DK. Vampiric doesn't give back enough HP (only 1 or 2 per hit), Carrion can only be used twice, and Sleep can only be used twice to help you creep only slightly stronger creeps. And he's totally helpless against spell immune creeps. Mediocre tank, doesn't recover his own damage very quickly, and requires a lot of Cannibalizing, which wastes too much time. So so, but no cigar.
The DK has Coil, which greatly lengthens the lives of Ghouls and can be used for on-the-fly high HP healing. In addition, Unholy Aura allows him and his units to move quickly from creep camp to creep camp and recover some HP along the way. He requires less time Cannibalizing than either the DL or the Lich, saving more time, and allowing more creeps to be killed, thus earning more items, more experience, and more gold.
Sorry I couldn't be more indepth, but Professor Winterchill's (ph.D in Undead Studies!
) fingers hurt like hell from the other one. Oh, and x42, I don't yell. I only remind you of the errors of your ways.
I'll be more thorough if anyone wants me to be (trust me, you won't like it).