I'm not sure about Amex, but I'm pretty sure you can use Discover cards at almost all merchants in China, as it works just like UnionPay. I went back to China this summer and basically had no problems with my Discover card. If anyone asks if it's a card issued in another country, just tell them it's a UnionPay card.
More details:
https://www.discover.com/credit-card...s/asia.html#q3