It uses MusicBrainz and TiVo (which is a superset of Allmusic) for metadata (credits etc).
Discogs would be nice and there are existing feature suggestions for it, but unfortunately the Discogs data is an unstructured mess that is making this difficult.
Wikipedia is only used for biographies and album/composition descriptions, optionally, in addition to TiVo/Allmusic.
While I agree that metadata can be spotty (but it depends on the label and the fan community), if there is something on MusicBrainz or Allmusic then it should be available in Roon. If it isn’t, this would be something to investigate.
Regarding the state of metadata in general, there are lots of preexisting discussions in case you are interested, e.g.,
Regarding Roon finding or not finding recordings of a composition, there are indeed annoying bugs. The typical issue is the one in this thread, see my responses and the links to other threads from there: