I make few purchases with less than 3%.
In terms of security, I've had my card cloned. Within minutes the bank locked the account and had a new card shipped. Hardly a big pain point they are solving. And most of us have more than one, so if any of our cards are down, we have others. It's merchants that currently carry the cost of low quality financial instrument standards we have.
For privacy, if I valued it over the $x,xxx I get back every year in the form of cash back, free insurance on rentals, etc, I'd use the most private of all instruments.... Cash.
Pretty mediocre card if you ask me. Of course, the cool factor will make this a success. They only need the right % of the cool crowd to move in on it, and the rest will follow the virality. Oh us humans.