My husband, 20 year old daughter and I had a lovely time on Harmony of the Seas! We've been on 8 cruises and by now consider ourselves old hands at it. I've always enjoyed reading the reviews on Cruise Critic.. but everyone is looking for different things from a vacation, so all I can do is relate my experience, and I hope it helps some planners!
We flew into Ft Lauderdale the night before, and a lucky thing because thunderstorms held up our flight for hours and we arrived VERY late at our hotel. We had gotten a 2:00 appt to go on the ship, but we got over there about 12:30 and no one had any issue with us getting on. It was a breeze- took maybe 20 mins till we were aboard. We sailed on the Oasis a few years ago, so I remembered that the Park Café was the place to go upon boarding (buffet on the first day is always a nightmare!) and it was an excellent call, hardly a soul there and yes!- the beef on weck was just as good as I remembered!
Speaking of crowds, my husband was convinced the ship was not full (although we did hear it was), because we did not observe insane crowds at any time. My theory is that because the ship is so huge, there are so many things to do and places to be at any given time, you rarely see an enormous bunch of people. The only time we were really aware of the crowds is on the elevators in the morning when everyone goes to the Windjammer, or when everyone is getting off in port. But an elevator you can get on eventually comes, don't sweat it.