MC uses YADB, and if no tracks are found, it uses FreeDB. That's pretty much the universe of metadata. The others may have still more entries, but it doesn't mean the quality of the metadata is better.
My experience is different.
Especially when tagging classical YADB in general return nothing.
FreeDB contains a lot but is incoherent.
The same compositions are often spelled different and/or in different languages.
Often the composer tag is not populated.
If there is a composer, it is often in the artist field and if one is lucky you can find the performer in the album artist or in the title.
AMG is far better structured.
The quality of the meta data is one of the reasons I do use dBpoweramp instead of JRiver.