Mouse+keyboard and controller each have their advantages in just about every game ever made. Controller has objecively better movement capeabilties and convientient button mapping, whereas M+K has more precise aiming and more assignable keys. Aim assist on controller is designed to mitigate the advantage of m+k aiming but is rarely perfect depending on the game.
In BF6, I have found that controller has a significant advantage in close quarters engagments because of what I mentioned above, while m+k has an advantage a longer ranges. I play on PC and use m+k for any gamemode where most of my engangments will be at 40+yds. I switch to controller for anything cqb. M+k has an enormous advantage when sniping though, to the point where they are barely even comparable.