They don't provide any backwards compatibility when adding even minor new features to apps like Notes or Reminders that synchronize through iCloud. If you update your iPhone but not your Macbook, new notes created on the phone may be silently ignored on the Macbook, or mysteriously vanish into the void if you perform one of the bulk operations that replaces the canonical list of existing objects with the older software's limited accessible subset.
I also know from experience that changing your Apple ID email address (which is permitted) will leave many iCloud services confused and trigger sporadic strange pop-up messages about your account status for years. I would not trust a merge button that needs to deal with disparate photo libraries (any of which may have existing byzantine compatibility baggage from the upgrade from iPhoto to Photos), Notes, Reminders, and all of the other services to leave a well-used account in a reasonable state.