The game will never be fair for everyone across the board all the time ever. That is simply not possible to do and anyone that lives and plays this game with that expectation will awlays be disappointed and it will never change either no matter how many come here and complain about it.
None of the guns in this game are OP they never were. Just players learn how to use them at their most effective means. That is by no means unfair or cheating no matter what anyone else ever wants to say about it.
This game was not meant to be fair and balanced across all scenarios all the time. It is meant to be off balanc by the simple fact of perks and attachments.
Shotguns are not broken, Snipers are not broken. They work as they are meant to, this game is not meant to exactly duplicate reality in how the weapons would work in the real world. So yes shotguns will kill in oneshot even to teh toe.
You can be killed by the knife in one hit to any part of the body as well. Something else taht is not realistic but has always been how the game has worked and never changed and never will be.