When creating a Git repository inside of a Dropbox folder using the GitHub application for OSX I get the following message:
Putting git repositories inside of a Dropbox folder is not recommended. Are you sure you want to create a repository here?
Why isn't that recommended?
If people are modifying files and using dropbox to do the syncing, you will likely run into problems.
If you use dropbox as a convenient way to give people read only links to your work, then I have found no issues with this and have been doing it for years.