For me the most and truest pressing issue is that cell modems are very, very tightly coupled with Android. It's still true for the Jolla Phone that it simply is a worse phone because the modem drivers are buggy.
replyMy impression (also for Ubuntu Touch, etc.) is that all these systems use the upstream vendors' Linux kernels trees and firmware blobs for Android.
Unfortunately, since we are not talking about Samsung or Google, but just some random Chinese ODMs, it's usually years old Linux versions and ancient firmware blobs full of known holes (e.g. the C2 is running a Linux tree from October 2022). It's only thanks to the tireless work of postmarketOS etc. that some devices boot on modern kernels.