Finally beat this and...oh boy if I have to say a lot. And not entirely positive.
Easily, by far, the worst Ninja Gaiden game I've ever played. The Game Gear one was much better.
The game looks pretty good, especially for its times. And the soundtrack slaps, especially because the madmen have put copyright-friendly Iron Man from Black Sabbath:
You know what was one of the biggest disappointments? The sword is a very temporarily upgrade. Few hits and then you return with punches-only.
The biggest frustrations were definitely Stage 4 for this (I swear,
cling into these branches didn't worked to me at all. I had to rely over and over with respawn and scrolling).
And then Stage 6. Oh god. Yes, we all loved the JoJo references there (Stone Mask and Whamoo/Pillar Man), but having those few checkpoints and enemies that one-shot was nightmarish, especially the semi-last 3 guys which had to kept being thrown away.
But still, yay: I did it.
Overall it was a 4.5/10.
Still better than Double Dragon, but it doesn't mean much.
Had a chuckle though when the game ended up and I got this screen: