The unified UI is my favourite thing about MCC; it's like a museum, everything standardized and in one place, Halo 4 alongside 1-3, it's easy to switch between games, the cross-game playlists. It makes it feel special and not cobbled together.Shame there seems no way to just release all the games as individual products at this point. Unified UI is just dumb.
If it didn't have it, they might as well just enhance the rest of the 360 back compat games for X and release H2A standalone.
It was co-developed by 343 and Creative Assembly. AFAIK, 343 has a second team that work on the spin-offs like H1A, Spartan Assault/Strike, MCC and HW2. They handled the art design and they've done bug fixes and balance updates for HW2 since launch.343 engineers snuck into the Creative Assembly offices and inserted code in a game they didn't make just to delete your saves.