AD units are NOT dispellable, however, that doesn't mean they are worth much. You can just run away from dispelled units, and even if the AD units are at your base, you can repair or run your workers. AD is good with Pact or Ritual however. I also agree that the time limit is way too short to make any impact.
The absolutely worst part about AD is how it chooses which units to revive. I mean sure 6 Tauren could have died and they become yours, but on the other hand, if 6 footmen and 6 priests die guess what... You will get 6 priests... with NO spells. I think this should be changed because I'd rather have 6 crappy tier 1 units like footmen, rather than 6 casters that can't do anything.
AD however, is good at countering Resurrection. You might want to consider using necros to steal bodies as well. So in total, I think at level 6, you may want to consider placing it into pact, if you haven't already.