What we like about having the Google app is that we get notifications whenever she adds photos. No need for her to text us to say, “Hey I just added Halloween photos” or “GD was being silly today, go check out the pics!” It’s more efficient and more fun than just trying to remember to go check the album every few days/week. What I DISLIKE is that Google requires me to connect my iCloud Photos to their app in order to use it. 
I have the $2.99/month plan for ICloud and I have a huge amount of photos and videos (25k/1500). Yes I need to purge! Checking my Google app, I have Back-up turned off. That’s why I can have so many more photos than @abasket but not be out of space yet (but I’m close and do get warnings.)