My knowledge on the subject is limited but from what i understand, HURD is a project that seeks to bulld a microkernel based replacement for the UNIX kernel. Contrasting with the Linux kernel, which serves the same purpose but is monolithic.

The difference between a monolithic kernel and a microkernel is that the first runs all the kernel services that compose the operating system. Whereas in the later, the kernel runs only the most essential services and rest are implemented by servers. At least theory, this makes the system more adaptable, modular, and robust.

I think it's pretty similar to the Systemd vs minimalistic init systems debate. Systemd does it all, but by doing so, it could limit your choices on the software you wanna use for each task. Also, since it has a lot of features, there are some that you'll never use, which is why some people consider it bloated.

Init systems like Runit seek to solve this by just being init systems, nothing else. Other tasks are handled by other programs that are similarly designed for just that one task.

Personally, I'm in the "systemd and the monolithic Linux kernel are great" team, but it's just a matter of preference ¯\_(ツ)_/¯.