It's easier to keep multiple yubikeys than multiple phones.

Also Google relying on Google for anything important/critical is a bad idea. The internet is full of people complaining that Google deactivated their account for no apparent reason.

There’s no difference, insofar as you might lose the Yubikey and not your phone.

I guess I’d recommend enrolling both your phone and a Yubikey as a 2FA.

It's not, and it's worse since there isn't a PIN or biometrics keeping it locked. Keep one in a safe for emergencies (like losing a phone) that can also unlock your accounts.