I don’t want to be that guy, but I’m pretty sure that any achievement that reads “Done…Unlocking” but refuses to unlock is an Xbox One issue, not MCC’s. Let me explain something that I hope will answer everyone’s questions (but will probably just incite a bunch of hate, unfortunately):
Achievements on the Xbox One are unlocked server-side. This is the reason that, unlike on Xbox 360, you cannot unlock achievements while offline. Yes, you can complete their requirements, and, once you sign in and connect to the server, the achievement will unlock. Unfortunately, these servers get really bogged down when lots of people are online and using Xbox One’s Live servers. This obviously happened on Day One of the Xbox One’s release and, of course, on November 11th, the day that a HUGE amount of fans had to download the 15 GB patch. Here’s the part which I’m not sure about, but I have a theory for it, at least. I think that, once the achievement requirements are met and a player’s profile is able to connect to the server, no matter how poor the conditions are, the profile sends a request to get the achievement unlocked. When the servers are as busy as they were, I’d assume that it would take quite a lot of time to receive the request and ping a response back. In fact, what may have happened is that the servers were so busy that many of the achievement unlock requests took too long to be received or responded to, and, as a result, were terminated.
So, now that we’ve got that bit of wonderful news, what can you do?
Well, I have a friend who had to wait 7 months for an achievement, but he never had to complete the requirements a second time, and the achievement did finally unlock. So, if you have a “Done…Unlocking” achievement, the only thing I can recommend doing is contacting Xbox support, which doesn’t seem to be helping much, or simply waiting until the one day when they finally unlock. Sorry there’s no easier solution. :-/
However, there are some achievements that do not unlock or even read as “Done…Unlocking” when their requirements ARE met. If these achievements do not change their status for a week after completing them, then those are, in fact, issues with the MCC. For more information on the current status of achievements in the MCC, 343i posted this nice detailed post in the support section detailing this particular subsection of issues.
I hope this at least cleared some things up for everyone. At least try to hold off the hate, maybe? :-/