How good is the coverage of Apple Pay in China? Obviously not as good as Alipay. But is it useful?
Pretty useless I think. The use of Apple Pay is covered on the Apple's website in China. The list mainly consists of supermarkets, convenience stores, and malls, as well as some internationally recognized brands such as McDonald's, KFC, and H&M.
However, according to a question - How is your experience on Apple Pay in China - on a Chinese Q&A site, many users reported that the shop clerks don't know how to handle Apple Pay even though the shops adopt the system. Also, most clerks in China don't speak English.
So I recommend to carry enough cash, or create a bank account and Alipay or WeChat if you really want to use the mobile payment (which I did and you can even as a traveler).
I found that in most cases, the cards issued only in China can be used in China. I added my Amex to the wallet, but when I tried using it at a supermarket, the payment was rejected. In fact even App Store China requires a credit card issued in China...