I agree it would be harder to find a FBSD admin then a Linux one. But, I disagree on FreeBSD being more complicated. There is only one FreeBSD distro unlike linux. The BSD's also do a good job at documentation. And the OS never really changes much. It's s nice and boring compared to linux which is more bleeding edge and quick to adopt changes. An amateur sysadmin would have been able to managed keeping 4chan running just fine if they kept doing the incremental updates and fixing the little things that broke along the way. But now its just to far out there and all its crusty spegetti code is public.
I hear the freebsd advocates say that as their criticism of Linux pretty often. But I'm not sure it's as big of a deal as they make it out to be.
There are a lot of different distros. But really what most people do for servers. Is either just Ubuntu, or Debian. That it red had, and they provide enterprise level support, and pay walled documentation for the people that end up going that route (imo just doing Ubuntu or Debian is the better choice because fuck Red hat). And even between all the other distros out there. All the differences are really minor. The only thing I can think of that people might want to use for a server, that would the people would need to learn anything new to use, is alpine, since it isn't using systemd, and it's got BusyBox instead of the gnu utils.
Really Linux is pretty much, you learn on one distro, you know them all, besides small changes here or there, and some edge cases that I wouldn't see people using for a server.
And Debian and Ubuntu, are very slow moving. Particularly debain. Which is the reason I think most people go for those. They provide a stable Linux base. Where you start getting a more bleeding edge system is arch, and a little less so, fedora.
All that said. Freebsd, is a really good choice for a server. But it's definitely got its particularities, and you definitely need to learn a good bit to use it effectively.