@skye another thing i haven't seen mentioned is there's no consistent boot environment or initial flashing mechanism. arm has uefi but a lot of android devices just kinda use whatever they're given

gpl compliance is a) "email the manufacturer and hope they poop something decent out" and then b) adapt their hacked up kernel to something that linux's usual facilities can handle, there can be a lot of scary work getting e.g. battery charging to work without the proprietary android shit, if you find yourself with a novel enough device/feature/chip you can end up misconfiguring a voltage regulator and killing some of the phone (though i haven't seen that happen personally) which is pretty discouraging

oh also the tooling tends to be shit and you can't get datasheets for a lot of things etccetc