- How does the build system integrate multiple, discrete repositories into a unified system?
- What are the dependencies between the repositories?
- How are the sub-repositories laid out on disk? Do the separate repositories use separate toolchains?
- How do I decide when to update the reference to a sub-repository? Are they completely separate? Versioned as one logical entity?
- Do separate repositories reference elements in each other? Can I copy files between repositories or should certain files live in certain repositories?
These and other similar questions exist when you use multiple repositories.
(I hope you see that multiple, discrete repositories aren't a panacea and there is a counterpoint to each of your points.)