U2F is a great idea but needs to be implemented with some key things in mind. A person is the ultimate and only true trusted entity. Duo very poorly handles the eventuality of a phone breaking, being lost, new numbers. This can be partially avoided for 90% of cases if people always set up 2 devices at the same time, more the better. 3 devices would fix 95% of complaints problems. However there needs to be a way to transfer to another phone after-the-fact, the technology allows, not the app.