Small, well integrated base system, with excellent documentation. Jails, ZFS, pf, bhyve, Dtrace are very well integrated with eachother, which differs from linux where sure there's docker, btrfs, iptables, bpftrace and several different hypervisors to choose from, but they all come from different sources and so they don't play together as neatly.
The ports tree is very nice for when you need to build things with custom options.
The system is simple and easy to understand if you're a seasoned unix-like user. Linux distros keep changing, and I don't have the time to keep up. I have more than 2 decades of experience daily driving linux at this point, and about 3 years total daily driving FreeBSD. And yet, the last time I had a distro install shit itself(pop os), I had no idea how to fix it, due to the rube-goldberg machine of systemd, dbus, polkit, wayland AND X, etc etc that sits underneath the easy to use GUI(which was not working). On boot I was dropped into a root shell with some confusing systemd error message. The boot log was full of crazy messages from daemons I hadn't even heard of before. I was completely lost. On modern Linux distros, my significant experience is effectively useless. On FreeBSD, it remains useful.
Second, when it comes to OpenBSD, I don't actually agree that security is its main selling point. For me, the main selling point of OpenBSD is as a batteries included server/router OS, again extremely well documented in manpages, and it has all the basic network daemons installed, you just enable them. They have very simple configuration files where often all you need is a single digit number of lines, and the config files have their own manpages explaining everything. For use cases like "I just want an HTTP server to serve some static content", "I just want a router with dhcpd and a firewall", etc, OpenBSD is golden.
i just thought it was GBH, but i'll update my definition if you can cite your sources...
Yes.