

If your privileged user doesn’t have a password, in some cases this could lead to any program being able to elevate their privilege quietly, unlike UAC.
I think this is the most important part. There really isn’t any protection against random processes trying to do some version of exec sudo $0 except for the fact that it requires a password.

My understanding is that most of this is down to ARM’s (relative) lack of standardization. Consumer ARM SoCs don’t even have ACPI, so you aren’t even guaranteed to be able to do things like powering off the system. Qualcomm pretty much has to add some minimum support for their SoCs to the kernel because most of their consumers will want to get Android working on them, but that doesn’t mean they’ll do more than they have to for that.
There’s a reason you can install Linux on any x86 PC and it will mostly work, but you can’t install an ARM Linux on a phone. Even Android forks like Lineage don’t support all Android phones, even though they’re shipping basically the same thing the manufacturers are.