The iCloud backup built into the iPhone for me seems quite seamless. As I mentioned in another comment, I have several thousand photos and videos that don’t fit on my phone because it’s only a 64 GB device. It’s been this way for many years. But I can still scroll back and see all the photos. And when I click one I view it instantly. I’ve never noticed any lag or any problems associated with the cloud backup. It feels to me exactly the same as if they were local.
First that’s not iCloud backup but iCloud Photo Library. Second, I’ve experienced data loss with exactly this feature. I stopped using it in 2021 after using it for six years from 2015.
