It is a hard game, and it does have some poor design choices. The most egregious one to my mind is the hammer quest.

You need the hammer to progress in the game, and the hammer is in Death Mountain, which is filled with these awful hatchet monsters. There's no way to block their attack (at that point), and they back up when you try to jump over them. The only way to really kill them without getting your ass handed to you is to use the downward thrust... which is in a town you need the hammer to get to.

That's a design error. Yes, you can eventually master the exact timing between their attacks, but that happens over a long period of time and usually with repeated playthroughs. It's a design error.

Having said all that, I love it regardless. It's one of my favorite NES games. It has its lumps, but there's just something about the fluidity of the gameplay in particular that keeps me coming back to it. If I ever got a serious inclination to make video games, one of the first things I think I'd make would be a Zelda II with roguelike elements. Same gameplay, different overworld/dungeon maps on every playthrough. That would make me one happy gamer.