I think iCloud cheated on its end of semester tests! It didn't deserve such high grade... the only thing it can always do correctly is iPhone backups, just that, like someone said there. It's slow, expensive and clumsy.
http://tidbits.com/article/17757
Increasingly the Google Photos iOS/desktop uploader apps offers a significantly more robust sync experience for me on my Apple devices.
The pricing is fine until you hit the 2TB cap (very easily done if you use Family Sharing especially), at which point you are hosed. Even offering the ability to pay $x per GB over this cap would be a lifeline. Again Google have much better options for this. That the default free amount of storage hasn't changed in years from 5GB for all your devices is arguably pretty mean in 2018.
Honestly, I don't understand iCloud for photos. I understand iCloud drive, it's a server. iPhoto's interaction with iCloud feels suspiciously like sync, so I actually have no confidence in it.