Sorry, I've never used the browser plugins. Personally, generally don't want code with squishy, hard to conceptualize access rights having more access to my web data than I intend, so I just run the desktop app or mobile app and use the copy password button to add it to the clipboard for 10 seconds.

Never had problems with connecting to the browser (as long as frequently you update the software/browser plugin).

IMHO The UI of the Linux app is ok, not grate but very usable. The Android app I think is worse though.

Failing to fill auto-fill is in my experience 100% to blame on the website in question. It happens more often then with some commercial alternatives. They just don't have the same amount of resources to work around websites doing stupid quirky things with password input fields. Which websites would stop doing quirky password input field stuff.