When I lived in Shanghai 6 years ago they were trialling QR for the metro. It was really bad, always huge queues with people struggling to get the right angle/illumination/whatever to make it scan. I’ve never seen similar issues with contactless methods and in fact I think SH subsequently moved to them. The amount of data stored seems pretty trivial to scale if necessary.
When I was in Shanghai this year I used a metro QR code via AliPay, and it seemed everyone else did too. There didn't seem to be any issue with scanning them so I guess everyone got used to that. There was however a queue by the security theatre bag scanning device at every single ticket gate.
Interesting, I didn't think you could pay directly via Alipay or Wechat.
Most cities you can use chengchema/dachema mini-apps in Wechat to generate the QR codes, but also all the old smartcards I had from previous trips worked in their various cities (but only work in that city) and usually had slightly cheaper fares than the QR-code ones, which were the same as the cash price to buy a single-use ticket.
But yeah, the bag checking is annoying in China. Shanghai is very relaxed compared to everywhere else - most places you even have to have your water bottle scanned to make sure it is actually water, or else take a sip from it.
