Animation cancelling is in almost every game available out today. I really wouldn't call it game breaking or anything like that, because it really isn't game breaking at all.
Even if someone AC's, if you're better gear than them and know how to play your class better, I think you really shouldn't have an issue beating them.
As for slidecasting via the mount bug with Engineers, I wouldn't even consider that ACing. That's a straight up exploit.