This seems incredible and I love the idea. Anyone using this for large projects? Any clear downsides or tradeoffs?

Not that I know of as I don't have a way track this. I did test importing and mirroring everything for Ghostty and Meshtastic projects, found out that initial import and push steps may take a while.

Great to have issues, PRs, etc in git, but requires each participant to set up a fork to participate.

No CI though, but an extension to capture job logs is on the roadmap.

Radicule does something similar. Also git-bug.