NG1 has a more "genuine" difficulty than 2 at higher difficulties. Spamming incendiary shurikens is not the good kind of difficulty that comes from good AI. Also skill in most hardcore action games involves finding the strategies and moves that are the safest-- in NG2 finding those techniques and moves practically breaks the game. They would do the same on harder difficulties, but as I said, it's hard to pull anything when IS are constantly sticking in you with no chance to dodge.
anyway, wrika, it's not just about a game being hard, it's any kind of challenge. SOTN is not hard, but finding out how to progress is challenging. NG2 on warrior isn't hard, but you still need a certain level of skill to get by.