Oh dear.
Criminals are aware of a technique of amplifying the radios of certain keyless key-fobs that unlock the car from a distance from your house. If plain old NFC based unlocking was used with iPhones and Apple Watches to unlock the car, would be another disaster waiting to happen.
On the other hand, Apple isn't that stupid and probably is aware of this and would involve authenticating using TouchID / Face ID to prevent this type of car theft anyway. But I'm still skeptical on the security of this.