@Kuro466 It's simply the fact that, like I explained just above, the PK (and EVA-8 to a lesser extent) can drop people before you can react. On top of it's ridiculously high damage per shot, the UI does a poor job of telling you how damaged you are if you don't have the time to take your eyes off your target to look at your hp. So while it takes people 20-30+ rounds (taking into consideration the missed rounds) to down somebody with any other non-sniper weapons in the game (with full purple/gold shield), the PK can down you in litterally 2 shots at the body even if you have max purple/golden shield (one shot = 110 damage). So while you can count in your head the amount of bullets you ate from say a SMG, AR or LMG and have a mental guess-timation of how much hp you have left without having to look at your hp bar, you cannot do that with shotguns because the feedback on your screen from eating a single pellet is the same as eating the whole buckshot.
On top of all that, if you watch https://youtu.be/9PfFPW9a90w you will understand that the game has TERRIBLE networking issues to the point where the fact that low refresh/tick rate (Hz) plus how laggers can be rewarded for having garbage internet (because as long as they see you, they can shoot you even though you would be actually behind cover for a while already) can lead to a LOT of "WTF" moments where you shouldn't have died or you should have killed somebody but it didn't happen not because you (the low pinger with good accuracy) failed somewhere but because you were facing a lagging Piece of S.
Put all that together and you end up with a fantastic amount of rage inducing WTF moments caused by weapons that dish a ton of damage per shot such as the PK and EVA-8 (and longbow/wingman/triple take but only with headshots which takes skill unlike shotguns).
Finally, even if they did fixed the networking and hp-loss feedback issues, the shotguns would still completely trump SMGs. Hence why they need to be toned down while the Prowler and Alternator both need to be buffed. The R-99 is good but requires a lot more recoil management and accuracy than if you're using a shotgun.
