Have to agree with @NickWB, have used Dropbox for years with RapidWeaver and have only had one issue, and it was my own fault, have the project open on two machines at one time. I can understand Realmac’s insistence of not wanting people to use Dropbox, as I am sure it is a nightmare for their support is something were to happen, but then that is on the individual, not Realmac for the issue.
If you are going to use Dropbox, be sure to follow the instructions I posted here, then you should have NO issue with it.