It is most certainly a nice project and i'm all for alternatives to Git, but for many people having to give up their current tooling (e.g. IDE integrations, specific tools for graphical graphs, interactive staging of chunks, rebasing etc.) would probably be a non starter, since it's another source control system entirely, instead of a front-end/enhancement of what Git provides (e.g. ticketing, wikis, merge/pull request discussions, CI etc.).
Here's hoping that Fossil has a nice future ahead of it, there is certainly a number of projects that would benefit from it's relative simplicity when compared to Git!
Also their docs are pretty simple to understand: https://fossil-scm.org/home/doc/trunk/www/permutedindex.html
Though personally i really liked the more visual nature of SourceHut's page: https://sourcehut.org/
The darcs FAQ from 2012 reports it as still unfixed:
http://darcs.net/FAQ/Performance#is-the-exponential-merge-pr...
(Yes this was real, and yes we hit it. I liked the patch-centric darcs approach, but that was enough to make us migration to mercurial, then later git.)
It's not for the general mass to say the least. You're better off using other focused products for each components than something that does everything in average ways.
The visual looks like they combined components from 20 years ago.