Hive Scum not having to collect ammo pisses many people off and greatly disrupts other classes’ already established playstyle. So the entire change to Pickpocket is basically subjectively motivated, rather than something that’s based on objective truth. People simply dislike a class that can be special and different from everyone else, one that can be completely free of picking up ammo clips like everyone else have to. It provokes them psychologically.
Psykers for example already have a much improved version of infinite ammo, such as the inferno staff that can pretty much outdamage any hive scum in the scoreboard in every harvoc 40 game despite the unnerfed Pickpocket talent. But nobody complained about Psykers not needing ammo, because it’s something most people are already used to right from the very beginning. Nobody complained about Psykers seldom having to melee, too. Even though they complain about Hive Scum seldom having to melee.