At Mozilla, we started with a monorepo, then went to a mostly-monorepo, then consolidated just about everything back into a monorepo, and have since "decayed" a

At Mozilla, we started with a monorepo, then went to a mostly-monorepo, then consolidated just about everything back into a monorepo, and have since "decayed" a bit to a mostly-monorepo. There were major gains from merging our source tree with the continuous integration support code and configuration. We've pretty much always vendored selected third-party code, so that didn't really change. Large collections of tests and their infrastructure have been much easier to manage as part of a monorepo. Given that our tooling efforts are mostly in handling a monorepo, I can't really judge how differently things would be if we had gone full multirepo -- my experience with multirepos has been pretty awful, but that's an unfair comparison since we intentionally haven't worked on tooling for it. We solved our worst multirepo problems by going monorepo, but I'm sure other projects have solved their worst monorepo problems by going multirepo, so neither really proves anything. The push for separate repos these days mostly comes from social reasons -- if you have a separable piece and want external contributors, that's a strong motivator for putting it on github since that's just Where Things Are these days. No matter how good your tooling and issue tracking and whatever is, even if it's far superior to github's, it doesn't really matter. People have to learn yours; they already know github's. I don't particularly like github's workflow, but I still use it.