Every sentence in this piece by David MacIver is gold and will resonate with anyone who’s had to deal with multiple intertwined repos. David writes:
When you have multiple projects across multiple repos, “Am I using the right version of this code?” is a question you always have to be asking yourself – if you’ve made a change in one repository, is it picked up in a dependent one? If someone else has made changes in two repositories have you updated both of them? etc.
You can partly fix this with tooling, but people mostly don’t and when they do the tooling is rarely perfect, so it remains a constant low grade annoyance and source of wasted time.
This is the voice of experience. If in doubt, use one repo. If not in doubt, still consider using just one repo.