I guess I will stick with Halo 5, because if every match is going to be infested with mouse players and the inevitable computer hackers and cheaters, then what is the point? I guess people don't want fair competition anymore, which to me, is the whole point of playing Halo in the first place.
I have good news for you, they pretty much all separate player pools based on input device, so you would only play against other controller players (I'm a controller player on pc btw)
Personally, I have no problem with playing against mousers because.... As long as the matchmaking system is good, I'll be out against players who are about my skill level (other decent controller players and mousers who suck lol)
Plus aim assist on controllers is a great equalizer. Hell, I'm BETTER with a controller on titanfall 2 than I am with a mouse because the aim assist is so strong for controller players.
The fear based arguments against mixing PC and console players are long dead and buried. I have a blast using my controller on PC in warzone, Apex, halo and titanfall.