You don't need a monorepo to have integration tests. You can very well have a CI that clones a bunch of stuff and builds and tests them all together. The issue

You don't need a monorepo to have integration tests. You can very well have a CI that clones a bunch of stuff and builds and tests them all together. The issue that monorepo is solving is regarding write operations: FOO depends on BAR, which depends on BAZ. If you need to change BAZ to develop what you want on FOO and they're all on multiple repos, you'd have to pull request your way from the bottom to the top of the dependency graph. This is what causes the friction that monorepos avoid, and this is the hard part of such workflow to automate with multiple repos.