It means that it's not fully compatible with the existing PC architecture in terms of aspects like power management, peripherals, etc. Skylake was the same to s

It means that it's not fully compatible with the existing PC architecture in terms of aspects like power management, peripherals, etc. Skylake was the same to some extent and it apparently caused really obnoxious, hard-to-fix compatibility issues with Linux too. I'm not sure those have been fully dealt with even now.