It's also significantly faster to clone after the repack:
time git clone --mirror https://github.com/drhsqlite/fossil-mirror
65.10s user 21.22s system 35% cpu 4:05.92 total
time git clone --no-local fossil-mirror fossil-no-repack
26.92s user 2.99s system 155% cpu 19.190 total
git -C fossil-mirror.git repack -a -f -d
time git clone --no-local fossil-mirror fossil-repack
5.42s user 1.18s system 211% cpu 3.121 total
Edit: took new measurements with --mirror on the first clone so that the "local" clones actually get all the branches.
I imagine fossils internal git export function is not robust. Even when you {fossil git export} the repo, you still get a much bigger .git than fossil (20mb more), with errors on some checkouts. Repacking doesn't do much either in this case, so the functionality changes.
Also: