Afaik a lot of bootloaders are proprietary/wonky, a lot of SOCs run custom bootloaders.
However if you do manage to boot things up, hardware with open-source drivers should just work, for example Jeff Geerling has couple of videos on youtube about running his RPi with external AMD graphics cards connected via PCIe, and it works.
However if you do manage to boot things up, hardware with open-source drivers should just work, for example Jeff Geerling has couple of videos on youtube about running his RPi with external AMD graphics cards connected via PCIe, and it works.