Things may have changed since then, but when I first encountered the project several years ago it seemed like the thing that made this project stand out compared to other player projects was a big emphasis on correctness and accurate playback. There have files I’ve encountered that for example VLC will play with quirks (color reproduction is not quite right, etc) that mpv plays perfectly.
Given how VLC is basically bulletproof and will play absolutely anything thrown at it - it's great to hear that mpv is up there ...
Playback issues... scrub problems mentioned elsewhere, audio/video get out of sync when seeking a lot; need to hit left arrow to reset, speed changes lag when changed a lot, when there's a short clip it can end audio early, etc, etc.
Not a huge deal, but if you use it every day for video and audio file testing/lyric transcribing these little bugs get old. The playback core needs a rewrite to be "preemptive" I'd guess.
I have a very similar issue with subtitles in MPV sometimes. Some unknown (at least to me) combination of codec, encoding settings, subtitle format, switching subtitle tracks, and seeking will utterly break it to the point that I have to exit and reopen the file. I can't be bothered to debug the issue (so far).
In general I don't understand the negative comments I see VLC get. I've never encountered major problems with either piece of software. I've encountered minor bugs and annoyances with both.
The media players we have on mobile don’t suffer from these issues, so they must be solvable.