The device keeps the secret in self contained memory and never exposes it. It is on the other side of a USB bus or in some cases NFC. There have been local atta

The device keeps the secret in self contained memory and never exposes it. It is on the other side of a USB bus or in some cases NFC. There have been local attacks against old designs via side channel attacks etc, but never once a remote attack. The model makes that pretty hard. The phone is essentially zero knowledge. Android and iOS allow arbitrary execution of user installed code and are a massive attack surface and have piles of pubished vulnerabilities. By moving secrets to very simple easy to reason about devices we get substantial reduction in attack surface. Also I have helped deploy these to several dozen people, taught workshops etc. It is no harder than teaching people to use Google Authenticator, but lower attack surface. Use U2F where you can and when you must fall back to TOTP at least you can promise an attacker does not get a free pass to genreate codes whenever they want which is something.