Which arguably is a good thing. As a owner of the system I really should have complete control over it. But currently there is software I have no control over running at even higher privilege levels.

The only modern silicon that gives me full control over what code is running is some (or most?) microcontrollers.

And this isn't just a question of FOSS principle. Especially SMM is problematic by unpredictably taking CPU cycles away from your workload. This can mess up hard realtime workloads, such as found in CNC controllers. If you are running something like LinuxCNC this something you need to measure to figure out if a given computer is suitable for that job.