Being a microkernel doesn't automatically make you more robust. If you look at their docs, Hurd has only one feature that's different to Linux and that's a sort of souped up FUSE. But as they realized later, allowing arbitrary unprivileged programs to extend the filesystem like that doesn't mesh well with the UNIX security model. You can write a "translator" that redirects one part of the filesystem tree to another, so you get the same issues as with chroot or symlink attacks. Their proposed solution (never implemented?) is to stop programs paying attention to translators by default if they are running as a different user. There are other cases where translators can DoS apps or do other undesirable things.

The basic problem is that the kernel boundary is a trust boundary. You assume that anything in the kernel is legit and won't try to attack you, which simplifies things a lot. In the microkernel world the original idea was that all the servers would be just ordinary programs you could swap in and out at will. But then the threat model becomes muddied and unclear. Is it OK to run a server that's untrusted? If so, to what extent?

The Hurd seems under-designed as a consequence. To make that vision work you'd need to be willing to depart from UNIX way more, which really means being a research OS.