My experience was that post-sync Apple Music “preferred” the online versions of songs to my local library, which meant that the extensive metadata I’ve applied to tracks, play counts, album art, and other modifications were no longer visible. When I realized this was happening and turned the sync off I was able to restore the affected tracks from my local HDD, but I would recommend setting up a discreet profile to sync with Apple Music and not subject your main library to the vagaries of the “matching” process. It’s also not very good at updates or deletions once you’ve done the initial sync. I’m sure the Apple Stans will weigh in with all the mistakes I must have made, or that what happened is actually the preferred outcome, but I consider myself a fairly advanced local-library iTunes/Music user (61k+ library dating back to 2008) so if it was this easy for me to run into problems, it’s too easy to run into problems.