Big Sur would not install problem free on my iMac 27 2017 (3.4 GHz Quadcore I5/1TB fusion/24GB). It hung halfway in a reboot loop, repeatedly giving the multi-language error message that something went wrong, wait for restart. After an hour I pressed shift, after which it continued and finished the installation and started Big Sur in safe mode. Performance was impossibly slow, peripherals could not be seen. Rebooting resulted in same error - iMac could only be started in safe mode, and slow performance prevented any practical use. Catalina had to be re-installed. Reverting to Catalina was also tough because the Time Machine restore would not boot, a clean Catalina install was required also. Next to the iMac on my desk I have a 2.4 GHz 2014 Toshiba laptop with SSD/16GB and Windows10 on a 27" UHD monitor, runs great - MacOS is beginning to be seriously less hassle free than Windows.

Lots of complaints over slow Big Sur upgrade - in this case iMac 27" 2017 24GB 3.4 GHz - impossible to use after upgrade installation on fusion drive, same if clean install on external thunderbolt SSD - had to revert to Catalina - no explanations yet?