I was in Rotterdam recently. Their metro system uses multiple approaches, some kind of RFID for hard tickets, NFC payment (without a ticket at all) and QR codes

I was in Rotterdam recently. Their metro system uses multiple approaches, some kind of RFID for hard tickets, NFC payment (without a ticket at all) and QR codes for time-limited tickets that you can buy in a phone app. Getting those QR code readers to read the QR code was a nightmare. Almost all the cameras have heavily scratched protective windows in front of them, which makes the reading process a struggle of trying to find a working angle. Of course the scratching is vandalism, but subway systems must be robust against vandalism, cause it's something that happens and must be expected. I switched to per-transit payment via NFC after using one of the time-limited tickets and realizing that you need to consider yourself lucky if you find just one working QR code reader at most stations. NFC worked like a breeze.