We’ve been on two cruises that used the medallion. The device itself worked well during embarkation and onboard the ship. It’s a good system, better than a keycard. The OceanMedallion phone app was a little wonky both trips, but we managed. Being able to order drinks from anywhere was a treat, but hardly essential.
You do need the app precruise to enter vaccination and passport data. If it doesn’t work right on your phone, try installing it on an iPad. And make sure you have the latest update (don’t be a dummy like me before our first voyage and suffer bugs that were already fixed).