I've been thinking some more about this - does anyone know if the U2F is compatible between different hardware wallets? You can restore the Trezor U2F to other Trezors, and you can restore the Ledger U2F to other Ledgers, but I've never tried to restore from one device to the other. Given that there are no open source third party tools you can use to restore your seed phrase to recover your U2F like you can do with bitcoin wallets, if you cannot restore to other devices that it puts you in a precarious situation of being entirely dependent on that one company. If that company goes bankrupt, or you are unable to obtain a replacement wallet from them for another reason, then you could find yourself locked out of your accounts.

If U2F is not cross-compatible, then there is a strong argument to made to always have a second device as a back up stored somewhere secure in the event of loss or failure of your first device.