Annoyance: Can't scan your music library from the PlexAmp app, can't scan it from the Plex app either. Super frustrating when music as added and you have to struggle with pop-up navigation on the Plex desktop site on mobile.
Game breaker: maybe it's just really hard to find and undocumented, but there doesn't seem to be a way to use profiles with PlexAmp, either to have individual play history and playlists, or to age restrict some music content.