Too easy to lose codes. If you get a new phone, you must use the QR code to transfer your accounts. They are not tied to Gmail. This is a very easy thing to miss since you are not warned until it is too late. Thus, anything that uses this for authentication must also come up with a way to recover accounts that get locked out due to this poor design, introducing a new, likely weak, target for attacking. In other words, this app only provides a false sense of security.