> in fact nothing at all apart from Raspberry Pi OS
How do you figure? E.g. OpenWRT, Ubuntu, Alpine Linux, Kali, and Zephyr all offer official image support. Others have unofficial support, e.g. I think FreeBSD actually falls in this boat.
A lot of linux native software example redshift still does not work on the rpi. Lot of things are broken. Recently i tried to use it as a basic desktop replacement until my new laptop arrives. Does a terrible f job.
A 10 year old celeron n2930 based mini pc i purchased for 30$ at a scrapper performs way better than the pi4. Ran esxi and bunch of vms on top of it aswell. Sips 10 w of power.
That's "easy" when all they have to do is to package https://github.com/raspberrypi/linux