I'm not too familiar with Parse, but Dropbox is not solving the same problem as the feature of iCloud that developers are complaining about. Whole file syncing is comparatively easy from the server side. If there a conflict between the server-side file and the client-side file, the app has to work out how to handle it. Core Data (basically a local database) syncing tries to handle conflict resolution automatically, since it has knowledge of the data structures. That's tricky, and that's the part that doesn't work well.

I haven't seen many complaints about iCloud's file syncing features, which are what Dropbox is the direct competitor to.