I have my first cruise with medallion coming up. If I had known a key card was still an option I may have opted for it instead. My concerns with the medallion have to do with its proximity based unlocking of cabin door. Sounds like a great idea if you have your hands full, but a couple of scenarios concern me.
1) It used to be if you lost your card, no worries. Someone finding the card would have to insert it in each door to see where it fit, which would look really suspicious. Now someone can just walk the decks slowly past the doors and hear the door unlock and see the panel. Yes I know, once reported the old one can be disabled, but there is still a window of risk.
2) When walking alone, maybe late at night, back to your cabin, if you are uncomfortable someone is following you, you could just keep walking to public/populated place and no one would know your stateroom. Now they may be able to determine which stateroom is yours as you walk past.
People can scoff, but cruises are like little cities and bad things unfortunately can and do still happen.