The security aspect is of course very important but so is developing a banking app.
However imagine the implications if this would work. Card readers would become obsolete and every shop could easily offer card payments to its customers since everybody has a smartphone nowadays!
From what I've read so far, the most difficult part would be to have encrypted communication between the phone and the chip on the card. But aside from this, the process seems to be mostly inter-bank communication which bunq is handling already anyway.
Maybe somebody from bunq with more expertise could comment on this?