Navidrome's killer feature is its simplicity, and not just the dead simplicity of setting it up: Jellyfin has everything and the kitchen sink, which is nice - especially for video, whereas Navidrome offers a well-honed, long matured, music search/browse/play standard API, namely Subsonic which opens a world of clients for a multitude of platforms and all tastes... On that front Jellyfin feels narrow in comparison.

Smart Playlists mainly. They let you add logical filters to create playlists. Think IF song_name NOT contains "live". That's not syntactically correct but that's the idea. Also lots of apps can connect to navidrome so you can import everything easily. Like Feishen is a desktop music player, and