iCloud Photos gets right everything that Apple Music gets wrong. Like Marco, I can imagine many reasons why Apple took a different route with music than the clean-slate approach they took with photos. I’m not in a position to judge what Apple should have done. All I’m saying is that the difference in results is stark. I understand the design and purpose of Photos (the app) on both Mac and iOS, and I understand how iCloud Photo Library is supposed to work. And, for me — and seemingly, almost everyone — that’s how iCloud Photo Library does work. You sign up, you enable it on all your devices, you wait for the initial sync to finish, and boom — now all your photos are available on all your devices, all the time.
Not one iCloud feature works reliably for me. Not calendars, not Reminders, not Photos, not Notes — nothing. In every single case I have items on one device that do not sync to my other devices.