The fundamental flaw with AR starts in Halo 3 and Reach was your inability to challenge a team once they’ve set up with long range weapons. And cross input play will lead to that exact problem, except the M&K team won’t even need to “control the BR spawns” to do it, they just spawn with longer ranged weapons than controller players from the get go.
The main culprit is their ability to laser beam players across the map with a Sidekick. Sure controllers can pretty easily out AR M&K players up close, due to its very high aim assist compared to the rest of the weapons in the game. But you can’t take advantage of that if you can’t close the gap. And as anyone who has ever been locked down in spawn by a well set up team in classic AR starts will tell you, you can’t close the gap if you can’t get out of your spawn zone.
I’ll tell you right now, I would never want to play a round of H3 if my team spawned with ARs and the other team spawned with BRs. Noone would. But that’s what cross input is like in Infinite.