Caveat: I’ve gotten 427/450 MCC Achievements, so I can speak from experience on these…
The following list of Bugged Achievements has been compiled for weeks in a List of Bugged Achievements Thread in the MCC Forums:
1) Stuck Achievements: Several achievements have been stuck for many users for up to two weeks or more. Stuck achievements occur when a player completes the requirements for an achievement, then that achievement shows up as “Done, Unlocking…” for an extended period of time. Causes for stuck achievements are unknown, and affected players are experiencing random achievements getting stuck. Most of the stuck achievements appear to be campaign specific, level-completion achievements. There’s been no official word from 343i as to the recognition of these stuck achievements or the status of a fix (If there even is a fix in the works…) it should be noted that the MCC is not the only Xbox One game with stuck achievements. Other recent AAA titles have had stuck achievements that eventually unlock anywhere from a couple of hours to weeks later, indicating that this may be a Microsoft server issue. The good news is that most, “Stuck Achievements,” appear to be a pre-20 November Patch issue; therefore the majority of achievements after 20 NOV 14 are no longer getting stuck for players. The bad news is that all of us still affected by these “Stuck Achievements,” have had no word from 343i as far as a fix goes, nor even public recognition that this problem exists.
UPDATE This particular bug affects the majority of MCC players and is highly controversial. Unfortunately there is no known fix to this issue at this time. Contrary to any suggestions; removing your profile, uninstalling/reinstalling the MCC, and/or hard resetting your Xbox One will NOT unlock these achievements. With any hope, a title update will rectify these issues…
2) Legendary campaigns in 3 Hours or Less for Halo CEA, Halo 2, and Halo 3 are completely bugged and unattainable for an unknown reason. Some me users have reported that Halo 4’s Naked Tyrant achievement is also bugged, but this one unlocked for me. I have 168/180 minutes on Halo 3 but nothing unlocked so that one is totally bugged for me.
3) Halo CEA LASO: An especially agonizing and game breaking bug, there are multiple accounts across these forums that in COOP mode on Halo CEA, the name plates and achievements don’t unlock for one of the partners; despite the fact that they’ve completed the entire LASO playlist with the same partner.
4) Halo 2A LASO: The current skull set makes this playlist impossible. Positive skulls would be appreciated but at the very least, Envy must return to Halo 2 LASO to make this feat possible.