Speaking from personal experience, it's an absolute pain -- they're already an odd duck (w.r.t. booting, for instance), add on that the documentation is incredibly lacking compared to competitors and you've got a platform that's not too appetizing.

Allwinner and Rockchip tend to be pretty solid, as far as SoC go; start here: https://wiki.freebsd.org/arm

I speak as a FreeBSD developer who has dabbled with RPi, having tested and shuffled in a lot of the initial RPi4 patches and did some not-insignificant hacking to get a single RPi image to actually support RPi3 + RPi4.