Something went wrong. It always does.
I had thousands of images distributed across over 60 shared photo streams. One day I rebuilt Aperture’s database and all the iCloud images were in one recovery folder. I deleted them and then most of my iCloud shared albums vanished.
This is a quick summary of how I recovered most of them from backups. I don’t know how this truly works, but it seems that this folder in my user account was a source of truth for iCloud photo streams:
/Users/[my user name]/Library/Application Support/iLifeAssetManagement
I copied what was there to an external drive then deleted it, logged out (necessary to close open databases) then logged in. With Wifi on when I launched Aperture it showed no images at first then downloaded what was in iCloud. So there was some kind of sync.
Next I did the same thing (closed Aperture, deleted, etc) but this time copied a backup of iLifeAssetManagement from prior to the bad event. I then turned off wifi.
On relaunch Aperture showed about 6100 images in “Shared:iCloud”. It rebuilt thumbnails for them. Then I turned on Wifi. Next I saw the count rise briefly as albums I’d shared previously came down from iCloud. Alas, the count started falling again, stabilizing at 5600.
I had most of my streams back — though one stream was much smaller than it used to be. Still, about 80% recovery and I didn’t lose a few I’d done post-disaster.
Better than nothing.
Sync without controls is truly hell (and Apple never provides enough control).
Update: Aperture shows 56 single owner photo streams (one is empty) and 5 shared. iOS Photos.app shows 20. At least one of the iCloud albums not seen in iOS photos.app cannot be found at its public link. The iCloud library and the Aperture iCloud library are not in sync. So I’d call this a failure.