We went to DL last year and printed out our hopper tickets to have a hard copy, as well as having them on the app. Day one went fine, scanned in and out of both parks no problems.
I kept the printed copy of our tickets in my backpack. It's a good thing too, because when we got to the gates the next morning one of the tickets was missing from the app. The CM at the gate had to call a supervisor to look into the issue. It took them about 30minutes to resolve the issue. The supervisor said it was easier and took less time because they had the printed copy of the tickets.
But what a nightmare! I will always carry a printed copy of tickets from now on for the Just In Case issue that may happen.