[0] https://vita.militaryembedded.com/5159-open-virtualization-a...
HURD is Mach-based; it is stuck in a pre-L4 world and thus irrelevant to this conversation.
Very 40 years ago technology. Still better than ~60yr ago UNIX, although they're trying to ultimately be UNIX-like.
It is sad Mach and HURD still keep coming up in microkernel discussion.
The story goes that the early days were dominated by some ridiculously bad project management that makes Cathedral and the Bazaar era "Cathedral" a nice and well maintained project.
Most importantly, GNU Hurd lacked the "ecological niche" to drive work towards it because it couldn't deliver enough, just like lack of information about 386BSD led to Linux (Linus explicitly said that if he had known about 386BSD, he would have worked on it and maybe forked it)
True microkernels are, alas, more of an ideology than a practical reality, as the long-suffering GNU/HURD team discovered; Tanenbaum has been clear that the MINIX/NetBSD experiment was more about principles than performance. That said, certainly many hypervisors have attained success with configurations that coincidentally happen to be the same footprint as a microkernel.