GNU HURD was hamstrung mainly by politics, not so much by technical issues. There was a ton of code written, but it kept getting ripped up and thrown out because someone didn't like it.
Note that I'm not claiming that some competent people could fix HURD right now if the political environment were better. It's more that the politics moved the project into the least tenable position. I don't know if there's a complete history somewhere, but just some things I managed to piece together from Wikipedia articles:
Berkeley wouldn't cooperate with development on the 4.4BSD-Lite modified kernel, so in 1987 HURD decided to go with the Mach microkernel. But then they waited 3 years for licensing issues to clear up before investing any real effort into it. CMU stopped work on Mach in 1994, so HURD switched to Utah Mach. Utah stopped working on it in 1996. GNU kept working on that one under the name GNU Mach. And then (from Wikipedia): "In 2002, Roland McGrath branched the OSKit-Mach branch from GNU Mach 1.2, intending to replace all the device drivers and some of the hardware support with code from OSKit. After the release of GNU Mach 1.3, this branch was intended to become the GNU Mach 2.0 main line; however, as of 2006, OSKit-Mach is not being developed.
As of 2007, development continues on the GNU Mach 1.x branch, and is working towards a 1.4 release."
In 2004, an effort was started to move to a more "modern" microkernel. L4 was the first and it died almost immediately. Work started toward the Coyotos microkernel, but between 2007 and 2009, focus shifted to Viengoos. But then "As of 2011, development on Viengoos is paused due to Walfield lacking time to work on it. In the meantime, others have continued working on the Mach variant of Hurd."
Is there any public statement or other public source for that statement? In particular, it would be very interesting to read about the concrete reasons which lead him to this conclusion.
Sorry, I got that from private communication with Thomas Bushnell, who was the principle architect on the project for a long time. His claim is that they had two choices, go with the microkernel, or start with the BSD code that was legally in the clear and rewrite the parts that were under a legal cloud at the time.
Thomas preferred the latter, Stallman the former. As events proved, the BSD approach would have been fine (particularly since the legal issues eventually got cleared up), while the microkernel approach ran into much larger unexpected roadblocks than anticipated.
In his own words: "Finally, I take full responsibility for the technical decision to develop the GNU kernel based on Mach, a decision which seems to have been responsible for the slowness of the development. I thought using Mach would speed the work by saving us a large part of the job, but I was wrong."
That's not him saying using a microkernel was a mistake, it's him saying that he was mistaken about how long it would take, and the impact Mach would have on development time.
