BSD base system

OpenRC and Runit work fine on FreeBSD in theory.

The problem is the 'base system' shit, while you can easily install OpenRC on another kernel than Linux as it's designed to only assume POSIX, you are hacking into the 'base system' and the way The dependencies on FreeBSD and other BSDs work is like with Arch, they assume the base system exists in an unmodified state which leads to lots of surprises and can break your shit with the next update.

The way modular unixlike operating systems where 'everything is a package' typically implement it is that dependencies are even properly encoded for essential system components so if the next update suddenly claims a dependency on such a thing you swapped out in an unsupported way at least you get a warning and the update won't succeed rather than succeeding and crashing your shit on the next reboot.

In theory anyway, they aren't always as good at encoding these dependencies properly. Arch is a good example as said, they often don't specify dependencies on shit where they're like 'surely everyone has this'. systemd on Arch is in the base group and some things which depend on systemd specify it, while others don't.