cross play eliminates that problem now. Provided the game gets acceptable performance.
Cross-play isn't universal. In Gears 4, as one example, it's only allowed for co-op. Multiplayer lobbies won't put mouse and keyboard against controller. If Halo 6 hits W10, I'd expect the same. I also HIIIIIIIIIIIGHLY doubt any possible competitive match in Halo Wars 2 wouldn't allow it.