Convenience seems to be a slippery concept. From my perspective, the convenience of mpv+youtube-dl cannot be beat. Neither the official web player nor the official mobile applications have the features I expect or desire.

For instance, I use text-to-speech to read aloud the subtitles of foreign language content. With mpv that's straight forward, but I wouldn't even know where to start with getting that to work using any of the official frontends of youtube.