Riot can only handle so much, and their game is constantly bugging in new and exciting ways each patch. Let's not add needless complexity where it doesn't have to be. I mean I don't want an 'ult by ult balance' for sylas for 140 champions. That's just creating a new Mordekaiser in the way of bugs.
I mean you didn't even talk about Syndra. Why not have it so he just has 5 balls for her ult?
He can never stun with Annie ult. Can it stun people since tha'ts normal for th eability?
If he steals brand ult, he doesn't get the brand passive procs, which is quite frankly most of the damage from brand ult.