Hilariously bad
It’s not actually possible to sign in on iOS 11. One is asked to provide the answer to a security question that isn’t actually displayed on the screen. If you switch back to your password manager to copy an answer in the hope it might be the correct one... the app takes you back to the start of the process. It’s also not optimised for larger screens; you get the oversize keyboard that says “none of us have ever tried this on a Plus device.”