When I worked on this was a few years ago, so NFC wasn't a viable option at the time. I think even today, some research would need to be done on how widespread

When I worked on this was a few years ago, so NFC wasn't a viable option at the time. I think even today, some research would need to be done on how widespread support is, we can't guarantee everyone has a brand new phone. but I agree NFC would be worth serious consideration now. When considering robustness of support, one nice thing about the WIFI approach is in theory you can fall back to just a web browser. So basically any device could be used for provisioning. And the audio method, would probably support a great set of devices. We didn't actually consider ultrasonic, just an audio codec that was in use at the time and doesn't sound awful.