I didn't say that dragurs weren't undead.
I am saying that she can raise dragurs yet you cannot.
Wolf queen raises armies of undead, yet you can only raise 2.
And I have said that there are plausible reasons for both of these.
Lu'ahs reanimation may require ridding the Draugr of their curse for consorting and serving with Alduin. Thus she may need to kill them first, then raise them. You can do the same thing.
Potema raised an army, yes. Do you want to raise an army? Too bad, as the other 10million + Skyrim players would cry "Unbalanced magic system!" if you got what you wanted. It is ridiculous that you cannot (or do not want to) comprehend the fact that Bethesda limits your abilites for balance purposes. It is idiotic to assume anything else.
For the sake of assuming, however, both of those necromancers had
YEARS of experience in necromancy (Lu'ah perhaps had 25+ years)