Perhaps from the implementation standpoint, but from the user standpoint it's second to none. For example, it's the only one I know that supports dual subtitles (--secondary-sid in mpv). For more advanced features, it also allows you to pass CLI flags to mpv, e.g. for things like `--video-stereo-mode=sbs2l` (subtitles and UI for 3D videos).

Also, enable "Run mpv in its own window" under Preferences -> Advanced. This removes all the issues caused by the default mode of embedding the mpv window (such as subtitles being on the video and not in the black bars).

Thank you for your work on mpv!