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.
I’m curious about what documentation is lacking? I’ve been collecting technical documentation on the RPI4 because I’m trying to understand the platform well, and maybe I’ve come across something that can help?
Pretty much anything that would need to come from Broadcom, really
The docs that most recently bit us are pcie/ethernet on the RPi4; I got in touch with Broadcom, who is now in a multi-year effort to figure out how to disperse usable docs (under NDA?) to the broader community. I lost faith after 9ish months of poking and getting back "oh, you know how it is" in response.
Allwinner and Rockchip tend to be pretty solid, as far as SoC go; start here: https://wiki.freebsd.org/arm