The chipset firmware will still be proprietary, and ARM chips are very often configured with a "secret" core that runs only firmware based code that is invisible to the OS. Add in the fact almost all ARM SoCs integrate modems that are themselves turing complete systems that have total system access and have priority control over the host CPU and you have less power there than you do being able to kill the IME and rarely run coreboot.
I don't care that such things exist, as long as there's a way to completely disable them. It should also be possible to verify that they are disabled, and be sure that they can't be re-enabled by rogue software.
The approach of Intel and AMD is completely hostile to anybody who cares about such things.
