When an add-on has twice as many 1 star reviews as 5 star reviews there's your sign it's likely not working. This is one of them. 54 "1" star reviews versus 24 "5" star reviews is evidence of that...
We can only hope the developer will fix it. Calling it junk isn't fair, but simply saying it's not working or isn't working suffices...

Doesn't work.
Junk

not working

trash. useless

Malware

use this instead:
"Downr YouTube Downloader"

В мобильной версии не работает. Обратите внимание.

Does not work. May be spyware

Good