I don't have a well-written argument against this practice and I know that many people do successfully use Git with repos/folders that also live in a cloud-synced location. So I suppose my answer is: if it's working for you, be thankful and carry on. But don't be surprised when things go wrong.
A hearty "yes" to everything that @rgaiacs just said above. I personally find it very frustrating that there is no concept of ".dropboxignore", for example.
And I have definitely helped individuals, in my Git teaching life, who had very concrete problems with Git repositories that a service like DropBox or OneDrive was also trying to manage.