But Meta's "hg" wasn't really FOSS hg for a long time and looks nothing like it today. It started as hg but was hacked to become what is now sort of Sapling (+ Mononoke + Eden + Buck + Phabricator + Sandcastle + Landcastle) integrated together.

The lore was that the git people didn't want to help improve monorepo performance, but the hg people were somewhat more interested so that was the reason behind hg. Then later, Microsoft got git people to upstream changes to accomplish similar performance improvements.

IMO, efficient monorepo dev cannot happen without an SCM-integrated synthetic FS. Also, efficient dev cannot happen without dependencies-aware testing and build caching. It further helps if dev tools are tightly integrated together and documented. Meta didn't do documentation well and didn't name things all that well either. Most internal tools also lacked man pages and generally depend on in-group knowledge hoarding rather than self-discovery and principle of least surprise.

Every now and then, some critical tool (to your workflow) group would come along, break their tools, and leave them broken for weeks with a flimsy excuse like "Oh yeah, our news people are rewriting it in Rust" rather than reverting to a working build.