> repo A branch B can only be built with repo C branch F, but all the other repos should be master
This alone is pretty much what makes me prefer monorepos. If you don't have a stable interface for all of your in-house dependencies (and nobody does early on in a project), you're doomed to spend a ton of time matching branches like this. Not to mention, a naive build process of "grab the latest everything and build it" will break in that period of time when you've merged the feature branch in one repository but not the other.
That comes with its own problems. You could end up spending 3 hours tracking a bug you thought you had fixed two weeks ago, only to find out that some other service is pinned to a commit three weeks ago.
This alone is pretty much what makes me prefer monorepos. If you don't have a stable interface for all of your in-house dependencies (and nobody does early on in a project), you're doomed to spend a ton of time matching branches like this. Not to mention, a naive build process of "grab the latest everything and build it" will break in that period of time when you've merged the feature branch in one repository but not the other.