Don't install the beta. iOS 9 beta literally bricked iCloud backups on my iCloud account. I had to get Apple Engineering to wipe everything. Even deleting backups wouldn't fix it, it had to be cleaned up in the backend. Yuck.
The production iOS 9 cloud restore 'bricked' iCloud photo sync from my devices (iCloud Photo Library, not Photostream which was fine).
iCloud Photos wouldn't update on the devices because iOS 9 thought the devices were still restoring from backup. Apparently a known issue with tedious workarounds involving mounting phone as a drive.
iOS 10 beta fixed the issue (perhaps expectedly, as it wouldn't make sense to preserve a pending iOS 9 restore after update), and all the iCloud Photo Library syncs worked again.
iCloud Photos wouldn't update on the devices because iOS 9 thought the devices were still restoring from backup. Apparently a known issue with tedious workarounds involving mounting phone as a drive.
iOS 10 beta fixed the issue (perhaps expectedly, as it wouldn't make sense to preserve a pending iOS 9 restore after update), and all the iCloud Photo Library syncs worked again.