As just one example it is often difficult to impossible to rebuild your code when a dependency changes. Build systems often don't expose this information usefully, so you'll find yourself either hand rolling something that looks at git changes and tries to infer what should be rebuilt and what shouldn't. Or you just rebuild everything in the entire repo every night. Or you can adopt Bazel everywhere which can be a hard sell in many companies but does give you better tools.

Another example: Most source code hosting products don't give good support for path based permissions which you need in order to ensure that folks with the right context review code before it is merged in. Which pushes you to segregate your repositories along ownership and responsibility lines.