@DuskHorizon you are right. Problems I have seen somewhat regularly over at least a couple of years:
1) Photos go out of sync between devices (I usually find it's a photo duplicated from a burst that gets added to my iPhone or iPad)
2) Shared albums stop updating with new photos
3) Contacts get out of sync
4) Safari iTabs continually have phantom tabs open on devices
5) Messages get out of sync
Reminders, Notes, iCloud files seem to work fine for me at least.
Right now, everything is fine except my iPad has had a phantom Safari tab open when no tabs are open for about a week. All the normal stuff failed to get it to go away - including signing out/in of iCloud/AppleID, and a complete delete and reinstall from backup, which used to work. I just turned the darn reporting of it off.