I was a recent adopter of Clef. I found it to be a very smart system for two-factor authentication. Ease of use was key, including adding additional URLs (like a staging URL) quickly, so it would work regardless of the environment you were logged into. I added it to a couple of client sites and they were wowed by the experience of not having to type a pass code and beat the 20 or 30 seconds you had to type it correctly. Some people might say that's easy enough to do, but if you're multitasking and someone calls you on the phone exactly when you're trying to type the numbers on the screen into the text field, the phone takes over and you either have to hang up on the person or answer and then toggle back to your two-factor authenticator app to grab the number before it expires and you have to type a new one all over again.