There's "the critique", as it's called: http://walfield.org/papers/200707-walfield-critique-of-the-G...

The paper was written in response to the porting effort of the Hurd away from Mach to L4, and realizing the latter didn't pan out well for object-capability systems.

Same people later wrote an experimental microkernel specifically for the Hurd called Viengoos, but that too fizzled out.