I think that there should be an option on manual tag for the user to select which criteria of the tag information (recording length, artist name, song name, album name, etc.) to look up when doing matches.
With the defualt, or all of the criteria, with the manual tag, I have done searches and found literally no matches on some songs, but then, when I manually, MANUALLY looked the song data up, by going to the MusicBrainz website, and typing in less of the tag information (or typing into fewer of the search criteria boxes), I found matches. (But, still, I had to manually type out the MusicBrainz Unique File ID myselft by minimizing and maximinizing the MusicBrainz website page with it on it).