For me its definitely got to be how bad the guns handle considering its suppose to be based on real world. plus pretty much all the nerfs explosives , class based , gun nerfs, stupid sniper sweet spots, etc list could go on.
Unrealistic Bullet Spread (Bloom)
Instead of bullets landing where your reticle is pointed with predictable recoil or within a normal, reasonable hit range the game artificially expands your cone of fire the longer you hold down the trigger.
This completely removes player agency. Even if you have perfect aim and track a moving target flawlessly, random number generation decides whether your bullets actually connect. Not only does this feel terrible, but it creates the illusion of bad netcode or broken hit registration, because you can literally see your sights on the target while your bullets ghost right through them.