I'm not sure if XNU really reaps any benefits from its arrangement. It's not so much a hybrid kernel as much as it is a chimera kernel - the OSF Mach component

I'm not sure if XNU really reaps any benefits from its arrangement. It's not so much a hybrid kernel as much as it is a chimera kernel - the OSF Mach component is used to define the low-level constructs like tasks, threads, VMM, IPC/ports (XPC sitting on top of it), system clock, host info and so forth, but no real reliability gains to speak of.