If after changing from mpv to mplayer and mpv again, a video fails to play I suggest to uncheck the option "remember settings for all files" in preferences -> general.

The Windows packages include the old mpv 0.11 because recent versions have problems with online streams, for example youtube live streams.

SMTube doesn't need youtube-dl. Youtube-dl is used by mpv for playing videos from streaming sites like dailymotion, etc.