I've also been really happy with Darcs, expect for two things:
- Conflicts are not handled very well, as if they were not properly stored internally. `darcs revert` on a conflict doesn't always do what I expect.
- The fact that it doesn't scale (even with the quadratic algorithm) means that big users can't adopt it, which means that too few people write tools for it (including a hosting website: I'm aware of hub.darcs.net, but there are many things missing, including on security). Darcs is hard to install on platforms such as Windows. Also, few people know it, which means that it's hard to collaborate using it.
I just set up Trac for Darcs hosting -- though it has some tension with the Subversion model -- exporting to git for people who want the pain. I'd investigate Sourcehut integration if I had the time.