What's your plan for when your dongle is lost/stolen/destroyed? Many U2F devices cannot be backed up and they recommend that you register with 2 separate devices. But while that is possible, it certainly doesn't scale and really is useless advice for the general case.
I have 3 U2F devices. My wife has 2. For any U2F logins we have, we have enrolled all of our dongles in each login.
At least one of each of our devices is always stored at home in a safe place, where we store other valuable documents. If either of us lose one device, we will buy a new U2F dongle, enroll the new dongle, and unenroll the old lost/missing/stolen dongle from each of the services we use.
You can't backup a U2F dongle but so far everywhere I've enrolled to use one it always recommends you enroll at least 2 and keep one in a safe place.
