I use a YubiKey to authenticate on some services on my laptop. But the only reason is because it is the only standard (FIDO2) that is kind of cross browser and platform. On phone apps you should be able to verify the user with a very high confidence by mixing input (PIN, password) + biometrics (fingerprint, face ID, iris scan etc...) + location info. All the necessary sensors and data are there, you just need to make good use of them. The only real advantage I can see is having a single (hardware FIDO2) key that can be used everywhere and regardless if the phone was reset or not (in which case you have to set the biometric stuff again).
Plus some Android phones can now be used as FIDO2 security keys (https://fidoalliance.org/news-your-google-android-7-phone-is-now-a-fido2-security-key/). So having another FIDO2 key feels like... having a security key for the security key?
But this is just my opinion. Please correct me if I said anything that doesn't make sense :-)
