Josh Marshall, prolific editor of Talking Points Memo -- one of my favorite public affairs pubs -- is a dedicated Apple customer: three Macs, an iPad, and an iPhone. He liked Apple's idea of keeping his Desktop and Documents folder contents in iCloud, so he enabled it.
That was a mistake:
. . . the operating system had decided to sync the iCloud version of my work computer desktop to my home machine -- not realizing they were two different computers. Seeing that something very wrong had happened I tried to uncheck the checkbox that started the process. There was a message that told me that it wouldn't erase the documents I'd already synced to iCloud. (Cool, I'd hope you wouldn't do that, Apple.) So I said okay.
When I did that everything disappeared. So all the files on my Desktop were gone. All the files in my Documents folder were gone. [Bolding his.]
In theory, it should be simple to coordinate the backup of folders from several Macs to iCloud: each has a unique Apple serial number and a unique MAC address. Either should be enough to identify individual machines.
iCloud isn't attempting to meld two versions of the same file stored on different machines. It is simply saying all the Desktop and Document files from Machine A are stored -- as are all Desktop and Documents files from Machine B. Different machines, different folders, different copies stored on iCloud. Kid stuff.
From his conversations with Apple support, Marshall deduced that
This was clearly an emerging issue, one they'd heard about from other users and one they didn't have a clear solution to yet.
For Marshall, Apple's tech support was very helpful, but he was forced to wipe the system, then do a clean install of macOS Sierra, and restore his data from a Time Machine backup, a process that can take many hours. He recovered his data, but not the time he spent fixing the problems.
The Storage Bits take
Apple is a client-centric company. For the most part, that is a good thing. But as its tortured history with online services attests, Apple often runs into trouble when integrating devices across networks.
I have no doubt that Apple engineers will figure out the problems with Sierra's new iCloud storage features. I expect they may also update their beta testing to take into account multi-device users like Marshall and me.
When it comes to new Apple storage features, I'd recommend doing what datacenter managers do with mission critical products: wait six months -- and a couple of updates -- for the bugs to get worked out before using them.
Sadly, Apple has shown, time and again, that niggling issues like data integrity and data protection just don't rank very high in the product requirements matrix. Until that changes, be safe, not sorry: backup critical data locally and to a cloud provider.
Courteous comments welcome, as always.