No issue using an American AMEX with SmartEX a couple of days ago.

My own frustration: after a few minutes of futzing, my iPhone wouldn't display the number I needed to link my digital Suica to SmartEX, and I was also traveling with kids who don't have their own phones, so I needed to get paper tickets for them to board. It turned out to be impossible to do this at the ticket machine because I needed a SmartEx smartcard, or something, so I ended up cancelling the reservation (paying a penalty) and buying new tickets at the machine. Screw this app, seriously.