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If you'd ask me to define the problem with microkernels with one word, that would be "complexity".

The problem with Mach, you mean. All the examples you listed are specific to it.

I don't know about other implementations, but I remember the original design of l4hurd (based on L4Ka), was even more complex. I'd same this applies to all "pure" multiserver designs.

Check out Genode.org, MINIX 3, or QNX. Seem to have gotten a lot more done than Hurd despite being microkernel-based OS's. KeyKOS is one of the best from back in the day with EROS being a nice x86 variant of it. Turaya Desktop is based on Perseus Framework.

Many working systems in production from timesharing to embedded to desktop that are microkernel-based. Hurd and Mach's problems are most likely due to design choices that created problems.

I don't know about the others, but at least both QNX and Minix3 cheated a little, i.e. allowing servers to write directly to other user space programs.

Also, the presence of microkernel+multiserver systems is still quite symbolic in comparison with the monolithic couterparts.

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