Bought a used OnePlus 7 Pro in late January 2021. It came with Oxygen OS 11 Beta, and had some severe bugs:
* Display issue on lock screen would cause mismatch between PIN/Pattern display and touch input making it very difficult to unlock
* Black screen requiring button combination to force reboot
Both happened several times a day.
At the time, I rolled back (easily) to Oxygen OS 10.3.8. When 11 was officially released, I ignored it for two updates. Finally, 11.0.0.2 came out and I caved. Guess what?
Same. Two. Bugs. Constantly. Also had a really bad issue during the install process that caused the screen to flicker rapidly the entire time.
Since this wasn't a beta, it was not trivial getting back to 10.3.8. Eventually I found the correct imaging tools and images and was able to get back to 10.3.8, which doesn't have any apparent or show stopping bugs.