Fossil takes the philosophy of the tortoise: reliability is more important than raw speed. The developers of fossil see no merit in getting the wrong answer quickly.
http://www.fossil-scm.org/index.html/doc/tip/www/selfcheck.w...
The website also makes the bold claim on the homepage that
In over two years of operation, no work has ever been lost after having been committed to a Fossil repository.
Considering that Git and Hg are as reliable as Fossil (AFAIK) but faster, this sounds like an excuse.
I've had git fuck with my code/data one too many times to fully trust it.
One great thing about Fossil is how easy it is to back up since it's a single file. I've always felt that million-file approach was clumsy, even when centralized in a single directory.
Using SQLite dbs for serialization is a great idea, particularly for prototyping. It's not quite as efficient as bespoke file formats and data structures, but frequently good enough, and getting transactions for free is a huge bonus.