I've run across more aim assist this weekend than what is normal. you take out the option to to turn aim assist on and off. You can still use aim assist with a controller. if you want to use a controller on pc, you should suffer the consequences. there is no way that a controller should be given an advantage over a mouse and keyboard.
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It really depends how effective the aim assist is and how good the person with gamepad is at aiming with one. I've seen some YouTube vids of players shooting so effectively with gamepad, which you can tell by the onscreen prompts, that you can't even tell its a person using a gamepad until you see those aforementioned prompts.
And I'm talking unassisted aim, because it's smooth. So for them, aim assist isn't needed, and could actually be a crutch if the aim assist is made too liberally fast. A good aim assist should never exceed the speed gamepad toggles are capable of. It really comes down to that.
Keep in mind too that some weapons handle way better than others, and weapons can change in handling according to what core components and attachments they have. This means some players will be able to pan their aim from one enemy to another quicker.
I play on Xbox one mainly and PC every now and then but with a controller when on PC. For both I use aim assist because it's there.
There is no way that I can compete with a mouse and keyboard. The speed and precision is markedly better than a controller even with aim assist on. Without it I wouldn't stand a chance. As it is I have to change my playstyle as things I can get away with on Xbox one is an instant head shot on PC