What does Hurd uniquely solve?
It's volunteer engineering; because they can! Also it is not possible to know how the fruits of innovation might materialise. There might be achievments coming from this in a serendipitous way. The Hurd is a process.
"Come and join the fun!" --Alan Cox
Linux wasn't going to big and professional, like Hurd, but better fun. Perhaps those roles have switched?
L4 is a family of operating systems, sharing an API, not code. The ancestor was built to prove that microkernels could be fast. SeL4 is a formally proved variant. They are open source. They are also used in embedded systems, like baseband processors.
Minix 3 is an academic project by Andrew Tanenbaum. A few eyebrows were raised when it turned out that it is present in all new Intel chipsets.
Then you have the hybrid ones from Apple, Android with Treble (classical Linux drivers are referring to as legacy driver on the documentation) and Windows is kind of hybrid as well.
As of Catalina, Apple was very clear that the long term roadmap for their OSes is to move all drivers and kernel extensions into userspace, which will be a gradual process.
As a supporter of microkernels I feel this is more a list of side effects. I'd put it this way: monoliths are ok so long as any critical code stays very small and comprehensible in it's entirety by an individual. It's also respectful of things that are not microkernels.
Good microkernels designs operate on the same principle, to achieve reliability and security they keep their critical code very small. It's not invulnerable to bugs, but it is well understood that minimising this surface area is the first step in minimising bugs, the secondary effect is also focusing attention due to minimising total lines of critical code. Microkernels are an attempt to take this to an absolute minimum by adding a layer of abstraction that makes otherwise critical code non-critical.
As a mere enthusiast I feel like this is the most useful lesson to take away from microkernels in other software, not triple redundancy or fancy reincarnation servers, but the fact that scale breeds complexity breeds bugs. Making sure the critical parts remain lean and inspectable helps a great deal in all software even when the separation is not as strict.
The Apple iOS devices all have L4 in their security enclaves.
Both of these OSes have open source options, though the ones shipped in the processors are closed.
The idea was a mach microkernel + daemons/message passing to do higher-level things.
Note the microkernel was also used in nextstep and now macos.