I see one limitation/problem.
Presently (as of Feb 2017), Spotify UID can be only be added to a MusicBrainz release, not a release group. Obviously, Spotify offers only albums as release groups. Example :
is also unique
So Spotify unique ID (i.e. 5NKTuBLCYhN0OwqFiGdXd1) should a seperate field in release groups.
Storing Spotify IDs in releases could lead to a terrible mess and millions of duplicates.