Version 1.9
The blocking bugs associated with the videos tab, playlist tab, and playlist's page still exist but refreshing on any of these pages can get the videos loading properly. If I had to guess, YouTube loads a limited amount of videos at once (while also flagging the watched ones) and if enough videos get hidden at once (all of said flagged videos at the same time), it screws up the automatic loading of more videos.

Version 1.8
Works exactly as advertised! Almost too good. Being new, it still needs some filtering features. Currently, the History menu is useless (hilariously hides everything as it loads) unless the add-on is toggled off (thankfully one click with the toolbar icon).

Also, the ability to whitelist channels (such as subscriptions) would be great, otherwise toggling the add-on (like with history) works. Although, this does allow easy viewing of unwatched subscription videos when they remain visible (see below).

Playlists are also finicky. The setting for viewing your channel's playlists works perfectly fine as a whitelist. The playlists tab of other channels though can be blank if some videos have been blocked from the videos tab. This blocking can also sometimes cause the videos tab to remain blank and not load any other videos unlike the situation with the History menu (see above). The playlists themselves are untouched if you're watching them, even if they have watched videos in them. If you try to view all videos in the playlist from its page, then it will either hide viewed videos or hide everything if you don't refresh (see the easy viewing I mentioned above).

One last curiosity about the History tab, sometimes it will fail to load anything after it's blocked all of the loaded videos. But, if I scroll within the sidebar, more history items will begin loading and obviously blocked after a second (when the red view bar loads on them). Unfortunately, this scrolling does NOT initiate loading within the videos tab.

While already a functionally great add-on, there are some conveniences that could be added for better filter settings.