I had this game as a kid. It was incredibly hard. Like harder than Mega Man 2 hard. But let me tell you it was satisfying to beat a robot master in it. It was definitely a game where the challenge was in how hard it was not in how you played it.
But it is easy to play it now and say "Wow this is hard and stupid! Look at these graphics! Lame!" But that lacks context. While I won't ever say this game was the epitome of PC gaming at the time, it was pretty standard video game fare.
Keep in mind that the NES, while less powerful overall than personal PCs of that day and age, could run games far better and with more precision than almost any PC out there. It was far easier to create a tightly polished game on the NES than on the PC. If it could look better on the PC than on the NES, then it was likely bleeding edge PC gaming, or what we consider to be AAA games in this day and age.
And more than likely the difficulty was more to hide that there was an insane lack of content. Three robot masters and very small levels, mostly due to size constraints. Things had to come on floppies (of either type), which were not necessarily cheap to make much less distribute.