Again, your conclusion is wrong.
a) Everyone can calculate AcoustID audio recognition patterns called "Fingerprints"
b) You can not calculate them with Mp3tag, but with MusicBrainz Picard
c) Everyone can submit such fingerprints with additional metadata like artist, album, tracknumber to AcoustID, not to MusicBrainz
d) AcoustID fingerprints can then be connected to MusicBrainz recordings. This helps people with very bad or not existing metadata to recognize their albums or songs.
The confusing part could be:
You can try to recognize your songs with the help of audio fingerprints (MB Picard -> Scan -> AcoustID).
If someone has already submitted the same fingerprint as you and connected it to a MusicBrainz recording, you get the matching Metadata back from MusicBrainz.
NOW the "not reliable" part is that nobody can check if your submitted fingerprint and metadata is 100% error free.
Some people care about data and submit only error free fingerprints and matching medata. Other's don't care that much.
That's the same accuracy like in other open source project, like Wikipedia. You will never reach 100% perfection.
If you see such an not yet perfect submission, feel free to fix it at the MusicBrainz side like here:
and unlink the wrong connected/linked/associated recordings.
And again I have to ask you:
What has this all to do with Mp3tag?
If you don't like MusicBrainz metadata, don't use it. If you don't like Discogs metadata, don't use it.
Feel free to add the missing data from other "more reliable sources" as @MotleyG already wrote.