Zappa was better in Slash. The power creep in AC did not do him any favors. IIRC they also nerfed his Triplets and reworked the sword in AC. But they buffed Roah back up to like #R levels. Zappa has mostly been the same in every game actually. It's just a matter of how strong everyone else is.
XX>Reload was balancing down
Reload>Slash was balancing down
Slash>AC was balancing up
Zappa pretty much stayed consistent the entire time.
If you really want to attack Slash. The best argument is that it's kinda boring compared to AC or #R because for a lot of characters they are really close to the #R version but weaker and it's not fun playing the same character you've played before if it's the same thing but worse. Which is definitely a valid complaint.
--
Whatever...