Not sure which “promise” was made, nor what is a CMO.
Picard is able to handle hundreds of thousands of files at a time.
When a collection was properly tagged, tags can be updated over the time without starting over. So, in this case, one may load a lot of files at once.
The limit usually stands between the computer and the seat. You, as inexperienced user, with thousands of unorganized files, are unable to manage more than few hundreds of files at once.
Especially when your collection is an “huge mess” to start with. Don’t expect the best vacuum cleaner to also wash your dishes.
I guess you are aware that Picard is a 20+ years old app, which is recognized as one of the best tool to tag huge collections of files, with a large basis of users with very different profiles. If it was that disconnected from its users the project would be dead since a long time, open source world has no pity.
That said, the UI (and the rest) can be improved, and we are constantly working on it.
As open source devs, we are listening to our users, but we also have to manage priorities due to limited resources.
You are welcome to help us, including suggesting UI improvements, for that you have to create detailed tickets with mockups of what you expect.
Of course, you can also donate to MetaBrainz Foundation.
To me, you expect too much from MusicBrainz Picard, that’s not a magic wand, but a flexible tool that actually requires some work from the user; and that starts by reading the documentation.
PS: MB stands for MusicBrainz (which is, mainly, the music database), the tagger program is MusicBrainz Picard (or just Picard).