>Of course, you could "manage" this complexity by attributing api boundary/layers, and these api changes are deemed to be important to not change too often. But that simply means you're a monorepo only in name - not too different from having different repos with versioned artefacts with a defined api boundary.

You have visibility into who is using what and you still get to do an atomic update commit even if a commit will touch multiple boundaries - I would say that's a big difference. I hated working with shared repos in big companies.