You can watch uBlock's logger when using the web player to see what it does. AFAIK the blocking rules are imported from EasyList.

However, as far as user experience goes, I don't share your experiences at all – the web player is almost unbelievably bad, with frequently malfunctioning (disabled) player keys, volume slider jumping all over the place and common problems when resuming playback after a while. (Also, the desktop application has more features, including the ability to select multiple tracks to perform an operation on, and naturally the support for media keys.)