Great news, I've been looking forward to this for quite some time!
So far I have only noticed one pitfall (which could be a big one) on the path from Buster (current stable) to Bullseye: pam_tally2 is no longer included as of a couple of months ago.
So if you have it enabled and upgrade, you risk locking yourself out of the system completely as all authentication will fail. The solution is to take the system offline and remove all incantations of it.
The upgrade script will ask you and attempt to remove it for you, but it does not always succeed in doing so.
Other than that, bullseye has been really stable for a long time already.
