It has its own pros. The biggest one is that it uses asymmetric cryptography. This means that the only one that can sign a challenge given by the server is the one that has the private key equivalent of the public key the challenge was used to be encrypted with. The challenge is sent to the client, in which the client signs the challenge the server sent with their private key and then sent back to the server. Since the server has the public key, the server can verify whether the signature is indeed from the private key owner. This is possible because the private and public key are mathematically linked. This is the reason why it is phish proof. Though I am not sure whether a phisher can just take the challenge, let the victim sign it and then give back the challenge to the server to login. Can anyone confirm that? They are not better than hardware keys (since they are basically software versions of physical hardware keys), but at least better than passwords. At least the breaches will impose much lesser risks when it becomes a standard.