I like the concept of a monorepo, but have found it challenging to implement because most developers are only responsible for their part - and there is often a big productivity benefit to keeping them narrowly focused. One trick, has been to have a monorepo for CI, rather than a monorepo for code. When one of the smaller packages gets updated the CI monorepo is triggered and all of the systems are tested for interoperation. Github makes this particularly easy with repository dispatches. It's been a wonderful "canary in the coalmine" for early problem detection. Bonus: The monorepo for CI becomes deployment documentation and can easily have its own set of tests that are specific to interop.
