Well, just off the top of my head, trying to use the Remastered soundtrack in CEA is easily broken. Dying, reloading a checkpoint, or restarting a level will switch the soundtrack to Classic.
Also, the last cutscene of Reclaimer in Halo 4 still looks like this:
Instead of this:
I understand that a lot people really only care about the MP at this point, but the long term preservation of these Campaigns in their highest quality form should be just as important IMO. I hope that one day all the Campaign bugs are squashed and we get 1080p 60fps Halo Reach in the collection.