One thing I gained in Debian Bullseye over Buster: Scanning to a recent HP all-in-one works out of the box. No HPLIP needed.
It would be surprising if Debian Testing were more stable than Debian Stable, and it is not. My in-place upgrade did not work. I did a reinstall. Things are working very well, but not well enough to do the same on my desktop, which will stick with Buster.
Debian Security team member worries Chromium has so many issues that it will be too difficult to maintain in Bullseye without more people involved. https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=972134#83