To be honest, biometric authentication isn't a bad idea for something like a lightning wallet. Bluewallet hooks into android's biometric system, for example.

The purpose of biometrics isn't really to provide a highly secure environment, its for convenience. If there were a shortcoming, it would be the false sense of security it affords. As long as people understand its not much more secure than the Schlage lock on the front of their house (que the Lock Picking Lawyer)

As we all know, the best security is combining something you have with something you know. But in the case of standing outside in -40 degree weather and in the blinding sun, you don't want to be messing around with 12 character passwords and yubi-keys every time you need fill your tank.

There is, of course, a fallacy with regard to biometrics. Its not really something you have, and its certainly not something you know. Anyone with a camera or a duster and some technology can fake your irises or fingerprints. Sure, fingerprint sensors have improved over time and are harder to fake than the original 1960's optical-only prism based ones, but nonetheless you leave your fingerprints literally everywhere you go.

Lets be realistic here, a hardware wallet regardless of its access-control safegards, is yet another false sense of security. Everything can be hacked, and absent actual encryption with a symmetric key that is separate from the device (like a password), there will always be a way to draw out the secret information contained within.

The primary use-case for a biometric hardware wallet is either to impress your friends (you shouldn't be flashing anything with a whole bitcoin on it in front of your friends), or to have a way of carrying around some spending-bitcoin with you but keep it separate from your phone. In short, it would serve as a single-app smartphone for crypto transactions, and you can do that with your old iPhone.