edit: one other important consideration is vulnerability of iCloud Keychain on iOS. Anyone who gets your iPhone's unlock code (say, if FaceID fails and you type it in manually) now has full access to your passwords if they can steal your phone. Apple has not seen fit to fix this security hole where that unlock PIN gives you access everything in the phone, including iCloud Keychain. 1Password on iOS,
I think for most people the reason to use 1Password or any password manager rather than iCloud keychain is cross platform compatibility. 1Password works with whatever operating system you’re running.
I do appreciate that it’s not vulnerable to this exploit though. I am kind of surprised Apple hasn’t patched it by now. 1Password on iOS is not vulnerable to this exploit either. Your choices are biometric unlock or the 1Password master password. Someone can’t use the phone unlock code to get access to 1Password. I covered the camera so Face ID could not work and it would not prompt me for my phone unlock code. Perhaps there’s a workaround that I’m not seeing