Some Big Sur startup volumes don't appear in the Startup Disk Preference Pane
In the past, the Startup Disk Preference Pane would list all available startup volumes, including CCC backup volumes. When Apple's APFS replication utility is used to copy a Big Sur System volume (something that is now required on macOS Big Sur), however, the cloned volume will not appear in the Startup Disk Preference Pane, despite being perfectly bootable.
Some SMB volumes can't support macOS sparse disk images
We have received several reports that macOS is unable to create disk images on SMB volumes hosted by various NAS devices. If you attempt to create the disk image in Disk Utility (for example), Disk Utility reports an "RPC Error". After months of investigation, we have concluded that macOS Catalina has more stringent requirements for sparse disk images than previous OSes.
Some Big Sur startup volumes don't appear in the Startup Disk Preference Pane
In the past, the Startup Disk Preference Pane would list all available startup volumes, including volumes cloned by CCC (whether CCC used ASR or its own file copier). Some Big Sur cloned volumes do not appear in the Startup Disk Preference Pane, despite being perfectly bootable.
We have reported this issue to Apple (FB8889774) and we are currently awaiting a response.
Apple introduced a bug in 10.15.5 that prevents the creation of firmlinks
The chflags system call no longer works correctly on 10.15.5 with regard to setting the special "firmlink" flag that establishes links between the System and Data volume group members. If you're establishing a new backup of macOS 10.15.5 or later, CCC 5.1.17 (and earlier) will be unable to create a correctly-functioning APFS volume group. Many folders on the destination volume will appear empty, and the volume will not be bootable.