Probably not the only one, but configuring your server for outside access is way easier with Plex.
Since I mainly use these services for streaming my music collection (long time cd collector), I declare that Plexamp is simply superior to jellyfin. It is really awesome and feature-rich and jellyfin does not even come close to Plexamp regarding music in my opinion.
Yeah, it‘s okay. But PlexAmp just plays in a different league.