I see two issues of the NFC tag + Shortcut vs Apple Home Key:
- There is no Express Mode, hence no power reserve when the iPhone has ran out of battery.
- I don’t know if Apple policy has changed wrt to security devices (that includes smart locks) but last time I checked it was impossible to simply unlock a door through an automation without answering a security prompt before. That means unlocking the phone, touching the notification and validate the warning is mandatory in this case, killing all the benefits of the simple swipe gesture of the iPhone in front of the lock with no need to even awake it.
PS: In my memory there was some hack allowing to bypass the security notification that prevents such complete automation, using a dummy HomeKit smart plug as a middle man to trick HomeKit by hiding the security nature of the script, but this again adds cost and complexity comparing to Apple Home Keys.