The technology is one thing but the only way to see if it actually works is by stress testing, and in City's case that is by using it for an actual event. They can simulate hundreds of people arriving around the same time, but they can't simulate the various different devices, phone cases, card clashes, people holding the phone to the wrong place etc.
I'm quite savvy with technology because I work in IT but even I made a mistake with a GPay purchase this morning. It got bounced because - I assume - I was not holding the phone to the right place, I was about 1/2 inch too low, NFC has to be quite close to the reader to work for security reasons.
It'll be difficult at first but the club and the users will get better at it over time.