This is wrong. If it's working, it may be just because YT hasn't rolled their adblock defeating code to you just yet. And other extensions may interfere with uBOL and cause it to stop working. Even there aren't other extensions running, the built-in tracking protection in the browser can sometimes trip YT's code. The uBO reddit post (written by the author of uBOL) points out that YT changes the relevant scripts twice a day, so even if you have a perfect setup, there are going to be windows of time where it doesn't work.

IOW, you can't just install another extension and make the problem go away. Your whole setup matters.