"Custom" would be the way Apple Notes or Evernote stores their data in opaque binary blobs. SQLite is standard and you can open it with plenty of third-party tools, but I see your point about plain text files being more convenient for certain tasks.
I sync Joplin via Dropbox across phone and 2 laptops. In theory a prob? But in practice rare to never bc im only one user on one device at a time. And im making small changes at a time. There was 1 minor conflict once and Joplin saw it and i quickly resolved it.