I found this for NetBSD from 2011 https://2011.eurobsdcon.org/papers/sonnenberger/fossilizing....

TLDR imported cvs2fossil, then fossil to git and mercurial. fossil was less optimized at scale. No dvcs was perfect. At scale things don't always import easily

"The number of artificial commits and conversion glitches could be minimized by cleaning up various issues. This was made easy by exploiting the database and writing small Python scripts calling “rcs” and related programs. Various tests show that the Fossil repository and CVS give exactly the same output for individual revisions. Differences when comparing working copies of specific branches are accounted for.

The performance of Fossil is competitive. Some areas like “fossil pull” need further work, but the majority of the work can shift to improving the user interface."