The biggest problem IMO with MusicBrainz is that there is no way to automate adding of new releases, so new releases don’t make it in as quickly as they could. Also it is CD-centric so not so good for Vinyl, mainly because adding accurate track lengths is more difficult for Vinyl than a CD release. Discogs covers alot of data that MusicBrainz does not but its schema is not nearly so good. The absense of things like bios and reviews does not seem such an issue, e.g you only need one bio per artist which can be derived from other sources, the key thing is coverage of releases which as you say is not as large as Rovi. I am surprised you say long tail isn’t good, that has not been my experience. (Totally agree about composer names in track artist I mention that here About album identification problems)
If money was spent by commercial companies on actually adding data to MusicBrainz then I think some of the gaps could be plugged. There have been attempts by MusicBrainz to deal with Record companies directly but this has failed as they are generally very tied into Gracenote.