To your questions:
Battery... my iPhone X2 is a couple of years old, it still manages a full day. I charge it every night. I doubt the app will use much power, but time will tell.
The QR reader is already in your phone software, using the camera. All the app does is grab information from the code and store it. You have to tell the App to read a code.
Anonymity: unclear, this is one point that concerns me. If I start getting a lot of contacts that I feel are incorrect I'll just delete the app.
