The first and last use case definitely feel that way. A smart home system would probably use a proper light sensor so that it will work if your phone is in you

The first and last use case definitely feel that way. A smart home system would probably use a proper light sensor so that it will work if your phone is in your pocket or face down or simply not open on the page that's controlling it. Of course, all of these issues apply to an app version as well. Controlling a character via gestures sounds like a job for the front-facing camera, not the light sensor.