And SUSE enterprise is another actually popular version, at least what I've seen in the telecom industry. So that's at least 4. Then, there's Arch which is pop

And SUSE enterprise is another actually popular version, at least what I've seen in the telecom industry. So that's at least 4. Then, there's Arch which is popular with enthusiasts, and Ubuntu has become sufficiently different from Debian that it requires thinking when switching from one to the other. Gentoo seems to be getting less and less popular, but it's still kicking. And yes as the parent post said, it's the unexpected differences that are stressful: different init systems, selinux vs apparmour vs nothing, different locations of config files, different whole /etc structures, that sort of things. The distros are diverging fast, and switching from RHEL to Ubuntu nowadays feels like it's a whole new system.