We have cruised on the Epic twice. Once in the Haven and once in a balcony. Obviously, the Haven is a premium experience, but the balcony experience was also fine. The most frequent negatives:
1. The bathroom. This gets a lot of negative comments because of the bathroom design and privacy. In my view, an overblown issue. There is a curtain to separate the bathroom from the rest of the cabin when in use. Also, I like having the sink outside the bathroom as I can shave or brush my teeth while my wife is taking a shower. Unless you are traveling with a total stranger, which is likely not the case, this is not a big deal.
2. The elevators. Only two sets of elevators with none in the middle of the ship. Neither my wife nor I are youngsters, but we typically take the stairs to our cabin when the elevators are crowded after shows and during peak periods. It is good mandatory exercise which is needed after 20+ cruise meals. If you are handicapped, or can't climb stairs, for any other reason, this might be an issue. However, even in that case, if you wait for the theater to clear and then board an elevator, that will typically work.
3. Crowds. It is a large ship, but not large by today's standards. We sailed twice on the MSC Meraviglia a 6,000-passenger ship. That is a large ship. The Epic is no more difficult to navigate than most ships. Obviously, if the ship is beyond capacity that makes for more congestion. However, that is pretty much a crap shoot from cruise to cruise. We've been on ships at 80% capacity and ships 100% capacity. You adapt to conditions and figure out peak periods and times and places to avoid.
4. Entertainment. Right now, the Epic appears to be going through an entertainment transition. I actual welcome the change. On most NCL ships there are two big shows that run over and over. Once you have seen those shows, there are fewer entertainment options. It appears there will be different shows every night during the transition. We prefer that option.
A cruise is what you make of it. If you get on a ship looking for issues, you will probably find them. Eight days in the Carribean in a balcony overlooking the sea, dining in great restaurants and seeing a show every night is a good vacation in my opinion. Will there be occasional annoyance, probably. All depends on how you decide to deal with them.