I think the best use case will be to use a yubikey with a password manager. That way it doesn’t matter what sites support the security key directly. You could also set up passkeys with the sites so that once you authenticate with your password manager, the login process is transparent. Once more sites support passkeys, anyway.
I bought a couple of yubikeys but haven’t fully implemented yet. When 1password has full support for using a security key in place of a passphrase, I will consider using them as my primary unlock method.
I have to say that the Google Titan appears to be better bang for your buck than yubikeys. The FIDO2 yubikey is $55 which is pretty pricey considering you will probably want multiple. I’d be really curious if there’s a strong argument against using the Google keys.
I think the best use case will be to use a yubikey with a password manager. That way it doesn’t matter what sites support the security key directly. You could also set up passkeys with the sites so that once you authenticate with your password manager, the login process is transparent. Once more sites support passkeys, anyway.