Just been re-reading the FastMail blog from July 2016:
https://fastmail.blog/2016/07/23/how...ity-keys-work/
Quote:
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The security keys are small and light (they look great on your keyring), but we recommend you register at least two, or have TOTP or a backup phone number as well; you don't want to be locked out of your account if you lose it!
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It would be interesting to hear from FastMail on this subject, if someone should rely only on Fido U2F and that fails for whatever reason, how easy or how quickly would it be possible for them to restore access to an account.