[tptacek]

L4 is incredibly simple. It is essentially (a word I chose carefully) the opposite of a complicated OS. It also doesn't really do anything.

If you have just a few extremely simple applications you'd like to run in an enclave, L4 is a good way to minimize the surface area between the applications themselves and the hardware.

If you'd like to host a complicated operating system on the simplest possible hosting layer: again, L4 is your huckleberry.

Otherwise: not so useful.

Note that if you just host XNU on top of L4, you might rule out a very small class of bugs, but the overwhelming majority of XNU bugs are contained entirely in the XNU layer itself; having XNU running on an adaptor layer doesn't do much to secure it.