Google has announced Brillo which leverages Android and Linux. But no, wait, now there is Google Fuchsia because apparently Linux isn’t good enough. On the pi there are a jillion variants of linux to use, but with glaring holes (general purpose gpu, docker for instance). ARM has mbed tho it is really targetted at low end devices. You can try Windows 10. And all of these require a lot of laborious tinkering.
To say nothing of the cloud, the services for managing ARM devices are even more nascent or non-existent.
You have to wonder what Softbank is going to do about this. A $32B hardware bet needs a corresponding software and services bet.
UPDATE: Nat reminds me that I didn’t even mention toolchains! Cross-compilers, QEMU, etc — there is so much fun in here!