I know for a fact that YTM still has poor metadata support, the only way to fix it is to reupload the entire file. That's a huge reason why I moved to a self hosted solution. I know there were multiple cases where the metadata was wrong and two separate versions of a song were tagged as the same track. But I never lost any data. I would recommend doing a takeout and exporting all the songs if possible.
I have to imagine Google HAS to dedup on the server side, otherwise even a 100,000 song cap would be too much data to host for free. Google doesn't play ads on uploaded tracks. Google stores not-transcoded music files and has since "Google Music". When GPM first came out, it was a storage locker and felt more like direct competition to Grooveshark than iTunes.
Record companies seem to have a very tight not just on the songs, but also the metadata surrounding them. No service really offers the "radio mode" or suggested tracks for local playback, from what I found. I'd happily give a company playback metadata if in return I got that feature back. I wonder if Pandora has patents for certain parts of music recommendation algorithms. Spotify and YTM can rely heavily on user-generated music connections, but maybe that is why it can't work for local/uploaded files.