Mach isn't a microkernel. For example, NeXTSTEP was based on Mach 2.5, which is a monolithic kernel. Mach 3.0 was the only microkernel version, and to my knowledge, no one has adopted it with any success.
Mach 3.0 was folded into XNU in roughly the same way that earlier versions of Mach were used, but with more of the BSD functionality ported down into Mach rather than hosted POE-style in kernel space.
So 3.0 was adopted, with success, but not as a microkernal hosting user-space servers.
