I'm not familiar with those other approaches, but customisable metadata is only half what makes the database filesystem beneficial. Can you specify folders using a query language (via a GUI or otherwise)?

So for some simple cases it works great.

I have my music, my music app queries the database, presents my with my music, tags them into playlists etc, great.

I have my photos. OK, so I want to keep my screenshots and the memes I downloaded from imgur separate, but those are fairly easy to special-case.

Now I'm writing a novel in my spare time. I got my text document, I got whatever Evernote/OneNote-style system for notes. I found this cool MMORPG map editor I'm using to keep track of where my characters live and go to work, does that count as a picture. And I recorded when I hummed a few bars because I wanted to add a song my character wrote when he was bored, and now it keeps shuffling that in my music. Hmm these things are getting difficult to associate now...