With movement mechanics as they are, given extremely high strafe acceleration, the proliferation of crouch spamming techniques, sprinting and sliding, thruster, repulsor, and grappleshot combined, playing on mouse and keyboard in high level competitive scrim matches is extremely unforgiving. Aim Assist has always been an advantage on MCC in Halo 3, Reach, 4, H2A, and so on, but the extremely twitchy evasive movement in infinite makes tracking targets consistently on M&K nearly impossible at a high level. Aim assist massively helps in this regard for the overwhelming majority of close-to-medium range BR fights, whereas M&K players are left to do all the work on their own.
The movement is seriously too fast. I can’t claim to be an Onyx player or whatever, but I am currently Diamond 3, and have mostly spent my time doing scrims and tournaments rather than grinding ranked. Personally i feel like crouch spamming is already an issue that needs to be solved, and while you’re at it, IMO, i think that strafe acceleration needs to be reduced, enemy players just move so quickly in a quite jarring way, having spent the last 800 hours of my time in Halo playing Halo 3 on MCC.