The one thing that sucks (and someone can correct me if I'm wrong) is that if an NPC friend is attacking an enemy and you trying to combo of his attack it wont work because that enemy already got hit.
Is that scenario correct?
That doesn't have anything to do with attackone. It's because by default all attacks set a very short invincibility period to the target on hit. You can shorten the period further with legacy Fastattack property or control it directly using the up to date attack commands.
DC