Late 2015 4K iMac now running Big Sur
I have been running Big Sur on my iMac for a while now having previously updated my MacBook Air. I'm pleased to say that I have experienced no real problems with the upgrade. I made sure as far as possible that all apps were updated to ensure Big Sur compatibility. The only one flagged as incompatible by the OS was Avira Free Antivirus.
I tried the free version of the newer Avira Free Security that supersedes the free antivirus but found that it delayed the appearance of the sign-in dialogue to such an extent that on occasions I thought that the Mac had frozen on start up. I then found that there was no uninstall option, which worried me as security software can leave lots of traces if one simple drags the app to the bin. Fortunately I have a stand-alone uninstall app that seems to identify all the various files and this got rid of it, after which I didn't experience any issues.
The other worry was my MS Office 2016 suite, which MS stopped supporting in October last year. To be honest I find that I can do most things using Pages, Numbers and Keynote but I'm still using Excel regularly and haven't as yet encountered any issues.
So, Big Sur is up and running on both computers and my next challenge, some time in the future, will no doubt be if I change my Intel iMac for Apple's latest Silicon ARM processor. Quite a way in the future I would think!