IIRC, Ubers can keep up with the boosted power level of unrestricted megas, while OU mons can’t.Hate to say it but this tier seems hardly different enough from regular ubers. It's barely an OM.
It should have been OU, desu. There's hardly a reason to use the vast majority of megas when you can just use an uber instead. Especially disappointing is that lower tier megas are still basically useless even with the removed restrictions of using one, with some exceptions like Slowbro who saw use in Ubers anyway and barely functions any differently.
Really, the only real difference I can see is that it made Salamence kind of overpowered and made Diancie one of the best leads. Besides that, building a team for Megamons is virtually the same as building an ubers team.
I'd absolutely petition for this game mode to be moved down to OU while it's still early in the month, so we can actually see mons like Ampharos, Sceptile, and Camerupt used in any sort of viable capacity.
I mean, come on, it's literally called MEGAMONS and most of the megas still suck.