> The main obstacle of achieving binary compatibility of kernel drivers is that many Windows kernel drivers do not follow the standard Windows driver communication protocol (ie. passing IRPs when you need to call another driver) and instead just pass pointers around and call into other drivers directly.

Does this mean drivers that are part of Windows itself or third party hardware manufacturers?

Because if it's the first one that seems shocking and like it was a shortcut that could end up becoming very difficult to maintain for the Windows team...

Microkernels definitely have the advantage of forcing communication via correct channels and processes.