There is no doubt about it. Using a mouse and keyboard is a big advantage in a FPS game, even bigger than it would be for a 3rd person shooter like Gears of War 4.
https://www.youtube.com/watch?v=tXsTV4npXps GO4 is fully cross platform but they ignore how mouse+keyboard gains you an advantage, this advantage will be much more pronounced in a FPS.
https://www.youtube.com/results?search_query=XIM4+halo+5 Xbox still has no input detection and there are people actively cheating using the XIM4 , it isn’t the same as hacking of course but it gains the player a huge advantage over a controller based player. To date, no XIM4 users/consoles have been banned.
Will Microsoft ever do something to address these issues? Halo 5 does feel more fun playing with a mouse/keyboard. The problem is dividing the populations usually leaves the PC crowd into a dead zone with not many people. The solution is for microsoft to release their own peripheral mouse/keyboard or to enable general mouse/keyboard support so Console players can play with PC players.