I'm not sure I agree with you on that one Enker.
It's a gray area, to be sure, but my initial reaction is that there is a difference between "Undead" and "Skeleton".
As far as I know, since the spell specifically mentions "Skeleton units", this spell is only usable on things such as Risen Warriors/Archers, Bone Golems, Giant Skeletons, etc.
Units such as the Cursed, and the Unliving are indeed Undead, but are not technically Skeletons if we go by the books.
But, I can see the other side's point on this one, as well, for both continuity in storyline and mechanics (Undead are Undead as far as Necro's go, it's what they do, and do best), and ease of rules keeping.
An interesting point, I'm interested to see how the discussion progresses.