Glenn Fleishman, Macworld:
iCloud Photo Library, when it fits your needs, is a great way to avoid having to manage where your images and videos wind up. You capture video on your iPhone or drag an image into Photos in macOS, and it just syncs everywhere while making a central copy at iCloud.
However, there’s one configuration I can’t advise, and Macworld reader Eric writes in with a question that prompts a discussion. He’s wondering if he could rely on iCloud to be his “main backup of images.” The short answer is no, but it’s not about distrust in Apple’s technical abilities. Rather, about the frailty of all material things, and the risk of putting all one’s digital eggs in one basket, no matter how firmly the basket-storing company is holding that basket.
Interesting read, some good insight on iCloud Photo Library.