TL;DR - Dropbox has some issues with syncing folders and files that have very long path names. If you're using Dropbox and you place a .PDX bundle into a folder that is already many folders deep in your Dropbox, Dropbox will not be able to sync it properly (but will also not tell you which file is the issue) and it will be stuck syncing forever.
Not a problem with the SDK (I think?), but just a heads up in case anyone is using Dropbox !
If anyone wants to see the full details about what happened and how to fix it: