That's simply not true. musicbrainz tagging is quite poor, to be honest. Properly moderated music torrents (not only what.cd) in general have much better and more relevant metadata for each release. Unfortunately, it's not that available and usually requires a human to be interpreted.
Discogs is vaster, but still quite disorganized. For rather obscure music you still don't have anything better than the metadata that comes with the torrent (and is required on all quality music torrent trackers).
I've just got started with beets. It takes a lot of work to organize a music landfill that has accumulated over 15 years, but it may be the best tool for the job - even if the job will consume man-days.
Incredible amount of data, versioned and CC0 licensed, great and diverse community of music lovers, strong ties with archive.org.