In every sync implementation I've encountered in software, it's wise to understand a couple of key points. First, sync is not a form of backup, even though it normally results in one's data being kept in duplicate on multiple systems... it's a way of slaving each of the multiple systems to each other in terms of the data that is sync'd. Second, when slaving systems to one another like that, bad things happening on any one of them (or in the sync system itself) can immediately ripple through to all of them and trash the data on each.

To prevent disaster when the second point occurs, it behooves users employing sync'ing of data or settings to be extremely diligent to employ separate, genuine backups somewhere within their multiple-system universe. Otherwise, this thread is exactly what can result.