On paper, it's great. You select the videos you want to delete, wait some time, then BOOM; all the videos you would have otherwise had to painstakingly delete yourself are gone.

However, after running some tests, I've discovered that the extension only works as intended if you stay on the tab the entire time. I was initially under the presumption that I could run it in the background while doing other stuff on my computer, but that's not the case. It will skip/forget to delete videos if you do that, so unless you plan to not be on your computer for a while, expect yourself to be staring at that progress bar.
If you don't, well, don't expect it to even perform well with a load of 10 videos; it will find a way to dampen your expectations of quickly getting that playlist under control.

My review was initially 3 stars until I saw the dev's response to CafeHaine:
Curb your attitude, pal. I wouldn't consider immediately resorting to putting customers under the bus as good PR. Your message to seitek also rubs me the wrong way. The extension has some legitimate issues that I ran experiments on, and they line up with what they may have been experiencing.

P.S. For the record before you go saying "what kind of experiments", I tested the reliability of the deletion system when monitored and unmonitored. Unmonitored, it consistently failed to delete even half of the 10 videos I would give it, whereas monitored, it was rather successful, although I have yet to conduct this test with a 100+ group when monitored.
(I also made an account specifically to criticize this extension.)

Important Note: Youtube has to be in ENGLISH or it will not work!

As of now, the addon does not work with a French UI.
Switching to an English UI is required, but once done, it works perfectly.
Thank you, drhoads.

(However, there’s no need to call non-native English speakers 'lazy' or 'non-thinking' in other reviews.)"

Do not work..

Absolutely great. My Watch Later list has built up to 1800 down to 500 and now up to 2500 after years of watching and it is frustratingly time consuming to reduce (as you can guess by going from 1800 to 500). I'm used to solving these types of issues myself but it's simply impossible to know an do everything. Loved it!

Worked perfectly. It took some time as I had hundreds of videos, but it did the work!

It does what it says it will. Straightforward and easy to use. Thank you drhoads.

Worked great for me! I had over 600 old videos on my watch later list; this got rid of the ones I didn't want quickly. It would have taken forever to remove them one at a time. Thanks for saving me some time!

It sadly doesn't work for me. It shows that it is processing, but it isn't actually deleting