I'd call that a straw man argument. "Mission critical" to me implies high security.
OpenBSD is probably the most paranoid *nix distro, where security is very important to their mission. Hell, they don't even support telnet because they're suspicious. VirtualBox? Nope--too dangerous. Changes to the codebase are strictly controlled.
Most supercomputer setups use their own particular version of Linux, as it's easy to port and, most supercomputers aren't web servers--they mostly compute.
And let's face it--most, but not all, Linuces use basically the same kernel code, so it's not as if they're completely different once you strip away the GUI.
As far as what's popular being an indicator; well, Ubuntu Linux, the last time I looked, was the most popular, but hardly the most reliable. But then many times more CocaCola is sold than Bordeaux wine; that doesn't make it better. Popularity is not an indicator of anything but...popularity.
So, my router and mailserver box runs OpenBSD; my desktop runs Ubuntu (actually Xubuntu) and XP runs in a VirtualBox session. I also use NetBSD and very old Debian (e.g. "Woody") as well as old Windows and DOS versions as (hardware) needs arise. Backups are done frequently, so I can afford to lose a system from time to time.