[dalke]

"Just as well" in all factors? What's the audit trail for smartphone app? Can modifications to the app be detected, or the app replaced with a real-looking fake app which favors the driver? Might the driver have two smart phones, one with real app and another with a fake app, and choose one based on the likelihood of not being caught?

Those scenarios are much harder to pull off with a "bulky fixed function meter device" designed to be inspectable.