There's one other thing to consider with necromancy. Liches. If you've never heard of them before, a lich is an undead sorcerer, a necromancer that has become undead and binds their soul to a phylactery, a vessel the soul can return to whenever the lich's body is destroyed. What's more, they often gain an array of offensive magical abilities to use, which can be anything from intense cold, diseases, or general evil stuff depending on the setting and persuasion of the lich in question.
This makes them a royal pain in the ass to kill regardless of the setting, because they can come back from any form of destruction to their physical body, up to and including disintegration. You have to track down and destroy the lich's phylactery, then kill its physical form to put an end to them. All the while, you have to put up with its undead minions and whatever magic it throws at you. Slaying a lich is not for the weak of heart.
An enemy like that would present a few unique difficulties in Dwarf Fortress. Up front, you're fighting an upgraded necromancer who is just as durable as its zombie minions and has magic attacks to defend itself with. Considering the lich would probably have the [EXTRAVISION] tag, and as such is able to see through walls, it would be able to raise the corpses in a fortress's graveyard without even walking through the front door. The extravision alone is enough to spice up an undead siege, but then you have the fact that the necromancer is supporting its minions with magic attacks. That's not a threat to be taken lightly.
Then you have the phylactery to consider. If the fortress defence force manages to kill the lich, they've broken the siege but the battle is not over. The lich's soul returns to the phylactery, it regains physical form and prepares to try again. That guarantees recurring undead sieges, with resulting recurring Fun, until someone goes and takes out the phylactery.
All in all, liches could make formidable opponents and a very worthy test of skill for fortress and adventure mode players alike.