Having played very little Ninja Gaiden, I don't think they're similar at all save from being 3rd person games. Nioh is much closer to the Souls formula, which slower combat, stamina consumption on actions and a demanding care on facing enemies, with the added complexity (though not necessarily added difficulty) of weapon stances and combos, skill trees, colored loot, and a Devil Trigger-like skill.
If you're going to try one, you should expect something more akin to Dark Souls + Diablo lootfest than the action romp that is Ninja Gaiden