Believe it or not, Winston is the worst character to use if you want to knock someone off the edge. He can only do it in his ult from, while Junkrat, Phara, Lucio, D.Va, Reinhardt and Roadhog have standard abilities to do the trick. That being said, there are some maps, depending on the control point that is up, you can really go ham and knock a bunch of people off the map at once with Primal Rage.
The mouse and keyboard would only benefit a few characters. Widow, Genji, and McCree would be the top contenders. It def gives people an advantage, but how much of an advantage isn't as big as it would be on COD, Destiny, and Battlefield. The character diversity of Overwatch and how much counter there is to projectiles makes it less noticeable. I would accuse someone of using it last night because everytime I knocked him out of D.Va's mech, he'd jump up and down to make him hard to hit while he had perfect aim on me. A practice that is widely used on PC. Anyone I see jumping around and shooting (with a character that needs to make precise shots) I'm suspicious of. You simply can't aim effectively like that with a controller. Side to side is more likely. It something I've done since COD4.