EMMS is the best music player out there (at least if you use emacs), as it allows full power and convenience of proper $EDITOR for music playlists and such.
All mpv backends for it that I'm aware of were restarting player binary for every track though, which is simple, good compatibility-wise, but also suboptimal in many ways.
For one thing, stuff like audio visualization is pita if it's in a constantly created/destroyed transient window, it adds significant gaps between played tracks (gapless+crossfade? forget it!), and - more due to why player starts/exit (know when playback ends) - feedback/control over it are very limited, since clearly no good APIs are used there, if wrapper relies on process exit as "playback ended" event.
What's curious is that I didn't see almost all of these interesting use-cases, which using the tool in the sane way allows for, and only wrote new wrapper to get nice "playback position" feedback and out of petty pedantry over how lazy simple implementation seem to be.
Having separate persistent player window allows OSD config or lua to display any kind of metadata or infographics (with full power of lua + mpv + ffmpeg) about current tracks or playlist stuff there (esp. for online streams), enables subs/lyrics display, and getting stream of metadata update events from mpv allows to update any "now playing" meta stuff in emacs/emms too.
What seemed like a petty and almost pointless project to have fun with lisp, turned out to be actually useful, which seem to often be the case once you take a deep-dive into things, and not just blindly assume stuff about them (fire hot, water wet, etc).
Hopefully might get merged upstream after EMMS 5.0 and get a few more features and interesting uses like that along the way.
(though I'd suggest not waiting and just adding anything that comes to mind in ~/.emacs via emms-mpv-event-connect-hook, emms-mpv-event-functions and emms-mpv-ipc-req-send - should be really easy now)