Streaming or player errors? Plex has basically never failed to play any video I've ever given it, but a bad connection can totally ruin streaming. I wish they would implement pause and allow to buffer.
In my experience Plex has a serious problem with determining whether or not it's capable of playing a file back directly without converting. I regularly have to switch it to "convert automatically" to make a file play.
What can be directly played and not directly played are just lists of formats that Plex bakes into the app, and not really anything to do with mpv, right?
Bonus trivia: Plex Media Player uses mpv as the video player