I was thinking on this subject, remembering my several attempts to beat that game, and here's my conclusion:
Ninja Gaiden is a difficult game, but its challenge is not adding. As I've written somewhere else in this forum, it's greatest challenge is to jump some holes without being hit by a craven - or a bat, or an eagle, or a missile, or a knife, or...
One can name a dozen NES games just as difficult as it (Battletoads, Castlevania, Bart vs. the Space Mutants, MegaMan 3) which are much more rewarding.
To my taste, if you remove the movie sequences, the music and the fact the main character is a ninja (we all love ninjas!), the game itself - i.e., the challenges, the stages, the bosses - is not a big deal.
Even the movie sequences, which were astonishing for their time, just reveal a silly and unappealing story. And even keeping in mind the NES limited graphic devices, one can just laugh at those enemies - specially the box fighter and the guy with a purple baseball bat!
That's my 2 cents.
Gustavo