Why do you care how Cosmos or DynamoDB or any other dependency is implemented? You only need to mock the interface to these services. Their internal code can change every day without affecting your tests.
And if you want to catch potential changes in Cosmos that modify the behavior of your own service, that isn't the purpose of unit tests.
Making stuff quicker is a good reason to mock stuff. So is not hitting real network services. But, in all cases, the best thing is to avoid mocking if possible.