The GNU Hurd has a lot of advantages, but there are challenges, too.

Some of these are explained in the critique.

Even though they're quite popular in the simpler embedded space, there is no successful true multi-server microkernel system for general-purpose desktop use yet. This is still an ongoing research effort. (TODO: add references.)

Likewise, resource scheduling in distributed operating system kernels is a research topic. For example, read more about it on the relevant Open Issues page. Also, the Viengoos research kernel project strives to explore these.

TODO: more to come.