Crysis PC allowed Servers with Controller users only. Sadly I don't remember many people using it.
Windows 10 WinRT API is sandboxed so I would imagine its harder to play illegitimately. Microsoft could easily lock out M/KB users from Cross-Platform multiplayer access.
The other issue I'm thinking of is PC players playing at 4k 120FPSThey see a lot more pixels and have a better response time than console players.lol.
That's the problem. The PC population would be much smaller if they forced controllers. One of the biggest draws of the PC is that aiming with a mouse if just so much more accurate and responsive.
Personally though, I don't think Halo 5 would work as well on the PC without some pretty serious changes. It's entirely designed with controllers in mind and it shows. Not that that is a bad thing in any way.