The Verge has a good explanation of why Google Photos isn't likely going to replace iCloud Photos for me anytime soon.
…Google automatically scans every photo and uses its powerful neural network to assign metadata.
If you trust Google then this is welcoming news — it makes life easier and saves time, not just in the Photos app but in services like Now (and Now on Tap). If you don’t trust Google then it’s just another case of the “don’t be evil” company exploiting you for advertising profit.
Remember, nothing is free. By using Google services or products like Maps, Gmail, or Photos you'll allowing Google to see, read, or scan every nuance of your life so they can sell that data to other companies. But Xavier puts it in a different light which really scares the bejeezus out of me…
…you now have to be afraid of those close to your that have Android devices. The seemingly normal action of taking a picture at a private party is now Google’s own private CCTV network in action. Photos will be uploaded automatically to Google’s cloud, analysed and categorised. Even if you choose not to use any of their services, they will be able to build a profile of you, based on photos and videos your friends take of you. Because they have facial recognition software running on their servers that is better at recognizing people than you are.
Think about that. Even if you don't use any of Google's products or services you'll be on their graph, because someone who does will eventually be linked to you through Google's smart algorithms.