Most of my production servers are running Debian “testing”, so I’ve already been running the Buster bits for a while. All seems to be working fine. Packages are only migrated from unstable to testing if they’ve gone 10 days with no severe bugs, so it’s actually quite stable. The software versions in srable Debian releases are generally too old for my liking.
Actually, the way I found out about the Buster release was that an apt update warned me thay the codename of ‘testing’ changed from Buster to Bullseye
I’ve never used Gentoo nor Slackware, but some distros don’t support in-place upgrades. I remember CentOS didn’t used to officially support it, so the upgrade process was basically “reinstall and restore your data from backup”. On the other hand, it’s so smooth in Debian! I have systems that have gone through several Debian releases over many many years, without any issues.