I'm trying to get a response to my support request on this same issue, too. Although, I didn't set it to synch with anything, it just automatically wiped all my user files from the main OS C: drive of my windows based laptop once installed and I'd logged in. So no internet issues involved in my case. From what I can see, recovery software indicates the user files and folder structure are, in theory, still there but I've retrieved very few viable files so far. They likely need repair. It didn't impact any system file areas, so computer runs normally otherwise.
If your drive is SSD, disable TRIM function immediately, and avoid doing anything that could cause your data to be overwritten including updates or continuing to work on the drive. I believe the iCloud app changed the file and folder markers to 'deleted' to permit overwrites, and the TRIM function can quickly cause loss of the data after this. You may need to take different steps for a non-SSD drive, so do look this up.
I'm truly shocked by this app from Apple. A warning it would do this, and seeking confirmation before doing so, was the least it could do.
