You kind of have to experience the worst of boths worlds to understand where and how each method works and breaks down.
With multiple repos it's harder for teams to share code and collaborate. Each team has a repo that becomes a little fiefdom where they are oblivious to who is using their code and how they're using it. Suddenly they'll push out what they think is an innocuous refactor and inadvertently break core functionality other teams took a dependency on for better or worse.
So what happens is the team with a dependency now copies the old code into their repo and take on all the extra burden of maintaining this old version, trying to backport fixes, etc. It becomes an enormous mess and time sink. No one ever has time to go back and fix things, and when they eventually are forced to do so it costs more time and effort than it would have taken to do it right from the start. You'll also run into horrible versioning problems where you're stuck on old version X but depend on widget foo which needs current version Y of that dependency.
You might say well bad on that team they should have engaged the product managers, made sure their dependencies and usage were well tracked with them, been looped in the process of changes, etc... but in the real world when your boss says X feature needs to be shipped in a few days all of that process goes out the window.