They offer a push authentication capability, so you only have to click "accept" or "deny" in the app on your phone. They've also got code generation and a hardware token as backups. In practice, I can usually authenticate through the phone in 2ish seconds.
Clef does look like an awfully nice user experience, though.
I don't want my ability to use a system conditioned on whether my phone can connect to the internet, or whether I have a laptop with internet connectivity and a camera that can take a picture of my phone's screen. TOTP 2-factor and FIDO* are better for that reason.
Clef sounds almost exactly like a software (totally app-based) FIDO implementation without a separate password other than the PIN you'd enter into the FIDO app. FIDO needs browser support, though; Clef can't do it that way, so it requires a data back-channel to Clef servers.
* (once it catches on... tokens exist, and Chrome/chromium 39 and above supports it, but MS has only announced it for Win10, FF hasn't implemented it yet, and hardly any major sites support it other than Google)
A lot of work has gone into the FIDO spec to make sure it can be used across a wide range of environments. I don't see the utility of buying into a proprietary 2-factor system that seems to be solving the same problem. If you added u2f-hid support to eliminate the hack of transmitting data via pictures and cameras, wouldn't your system behave exactly like FIDO?
Isn't the natural progression for a company like Duo or Clef to pivot to being a managed FIDO 2-factor service, for organizations that want central management of 2-factor auth without creating their own management system?
I grabbed an address from their website and asked if this email was legit or a phishing attempt and it took quite a long time to get a response. By that time, no one at the company had any patience left for Duo. You don't sell to enterprises by acting shady.