@Robert: Power failures are unfortunately one of the most common causes of this behavior since we have these JSON files open throughout the day under standard operation, and when our access to them is terminated improperly we end up with damaged versions of the file.
It is safe to restore all of the JSON files, but any changes to folder lists, contacts, calendars, etc since the day of that backup file will be lost during that restore. The only place I can see this truly introducing trouble is in synced clients.
For example; if a user has an Outlook client syncing an account via MAPI, the user adds a folder, then the files become corrupt and have to be restored to a previous version, this could result in the Outlook client trying to sync this missing folder repeatedly. To correct something like that we'd just need to have the user do a full re-sync (including removal of the sync key in Settings>Connectivity>Synchronized Devices) then allow the customer to recreate their missing folder once Outlook is back in sync.
Technical Support Specialist