What happens if the Yubikey goes bad? I use one for work, and the last 2 keys I had developed some hardware issues, and stopped responding, so I had to get a new one.
There's also a WebAuthn extension in the works to at least make it easier to maintain a backup key by not having to pull it out of the safe every time you register MFA with a new service:
https://www.yubico.com/blog/yubico-proposes-webauthn-protoco...