There also seems to be an impermanence to commenting on a video, especially one that's popular, because responses (unless sufficiently up-voted) don't stick around, and there's no way to easily search through them.

So a YT comment stream takes on a party line vibe and (IMO) there's not much point taking the time to write something thoughtful.

Mainly when I have commented is on videos on niche subjects when I'm reasonably sure the video poster/artist/producer will see it.