Is it safe to uninstall Big Sur?

yes it is safe as long as you have a backup of the previous system or want to install the OS that came with your Mac originally using recovery mode. I have reinstalled Mojave several times after installing Big Sur without any issues.

Yes, if you have backups. Notice that apps like Photos irreversibly update their library so you must have a backup of the previous library.

I've been using it since the second beta and have only minor issues that were easy to fix.

It is safe unless you backup with time machine