MnK player here. People complain about controllers because they have aim assist and reticle stickiness which makes aiming quite easy, compared to MnK which is mostly raw input and less consistent. In some situations, controller has an objective advantage over MnK. For example guns such as Auto rifles, snipers, and last word are objectively easier to aim and are more consistent on a controller. This leads people to complain because you can have an easy time on a controller with a gun that is harder to use on MnK. MnK has its advantages as well, presenting similar situations in which you can pull something off that you cant do on controller, but it is mostly related to movement whereas controller makes it easier to aim which is a bigger game cha-blam!- in a shooting game. I personally dont think the difference matters enough to warrant the complaints, I have reached the point where I don't care and I recognize that both are relatively pretty easy. However I can't lie, I use a controller once in a while for fun and am always shocked by how quickly I can pick it up and match my performance on MnK which I have used exclusively for the past 3 years or so.