I have the v6 NFC band. The Cards sub-app fails to add a bank card both in older and recently updated versions at the "strong authentication" step, or whatever it's called. It is even unable to show the list of supported banks, while the older version was at least displaying two placeholders (without real bank names) for my region. I got fed up and installed Xiaomi Wear instead, and could add cards without any problems. Looks like the main advantage of Mi Fit over Wear is syncing to cloud where the data can be accessed by Google Fit and other apps... however even this is unreliable. The updated version, too, is losing Google Fit account after the app closes (must add the account again every time) and therefore regular "preparations" are needed. I also didn't find a way to force sync with Google Fit and look at some progress bar until the operation really finishes. Let's hope the app will become better in some 6 months.