I looked into this a year ago and most sites did not offer to register a second key, so if you lose your key, you can kiss many of your accesses goodbye. I would never have the key to my digital life on a keychain... The idea is good, but it will cause huge damage if you lose your HW key. On the other hand, if you are cautious and use different PWs and a password manager with 2FA, you are quite safe.