The main reason I wanted this extension was to make Youtube stop spamming already watched videos, random playlists and useless shorts.
1) It doesn't block already watched videos, it just makes them less visible by hiding the thumbnail and dimming their links;
2) Sometimes it marks random videos as watched, but I never even clicked their links, even less watched them;
3) It cannot block playlists, as far as I used it;
4) It blocks shorts successfully AND entirely (not just dim their links).