I was appalled when my transit system recently introduced mobile fares, and I let them know exactly how I feel, as a designated early tester.
First you gotta install their atrocious little app, which I've always avoided because it was buggy and Google Maps gave the same real-time info in a better interface. So then you purchase some passes. They're QR-based, they're not NFC. Ugh, why?
Then during your entire voyage you'll need to maintain both connectivity and battery life. I hear tell that NFC can even survive a dead phone battery, but not a QR app. I was pissed about the connectivity, because I tend to disable networking while traveling, and I have a 1GiB data plan. Also, it makes for one-handed boarding because they mean it when they say "Have Fare Ready" so I feel like fiddling with my phone onboard is like fishing in my pockets for a pass, but worse, and I've already had passengers complain at me.
Now they're revamping the fare system again, and they're introducing NFC-card-based payments as well as account-based rather than pass-based fares. The punch line is that the mobile app won't be compatible with the card, so you need to choose one or the other, permanently. Guess I know which way I'll go.