Many users complain on SUMO that they have lost their profile data, assuming that Sync is an online backup service.
They are used to this in other browsers, so they don't think that it is necessary to have Firefox Sync on another device or that they need to back up their profile manually before formatting their computers.
Explaining this in KB articles is not enough, few people read it.
Firefox needs to make this clear through backup suggestions in popup notifications, banners, whatever. This could be done whenever the profile has been "changed enough".