a security nightmare, phones are updated from 2 to 4 years and then left for dead. after that no more security updates and apps will eventually stop supporting your version.
also, android lacks proper networking tools. no root access is also a big problem when selfhosting.
oh and let’s not forget the unlockable bootloaders that will lock your “server” into an Android version full of spyware and whatever crap the manufacturer decided to include.
but even on a a clean Android like Lineage OS, Android is unsuitable for an OS because it’s ultimately designed as a phone OS which are too restricted for self-hosting or doing serious computing at all.
they also have zero reparability and upgradability. if my storage runs short, I just buy a larger HDD and replace the old one. Cloning the disk is as easy as using one command, good luck cloning your whole system in Android. If my CPU dies or I need more processing power, I buy a new one and plug it in.