I have tried the new sync facility with a small database and carried out the recommended Verify & Repair, which reported no errors. The database has only 16 groups and is only 7.2 MB. I chose to sync it to Dropbox. No errors were reported, but bizarrely at the end of the sync process one of the Groups had been replicated. I recovered a version from Time Machine that was made just before the sync, and confirmed that no replicant Group existed prior to syncing with Dropbox. I realise that I can delete the replicant group, but this gives me no confidence in trying to sync much larger databases. I am going to hold off for the moment.