For PCIe indeed, but that’s not when the issues are present the most. There’s no standard register interface for integrated GPUs, modems…
> but it's very much against the original "spirit" of ACPI
Yup, it’s what Device Tree does too however, shifting this to the OS.
Another downside is trying to have a good driver-less boot scenario when PEPs are used, for the system to be able to go far enough until drivers can be installed. (N/A to Linux which is hostile to not in-tree drivers, but very much a concern on Windows)