One review said that Master Ninja was at least as hard as previous games, but didn't say which difficulty setting from NG and NG2 it was comparing to. It sounds like NG3's answer to increased difficulty just means more enemies at once and stronger enemies appearing earlier in the game.
I believe that NG3 has five or so difficulties, though I may be misremembering. I think they basically have Easy, Normal, Hard, then Master Ninja and Ultimate Ninja. Hero is, apparently, insanely easy, as the game auto-dodges and auto-blocks for you. From what I've heard from people already playing the game, Master Ninja is difficult because of the game's design; there are more enemies, and three fights into the first level, they start doing dodge-grabs. Since you can't heal during fights (no items) without the one ninpo you get (which, naturally, isn't available until later), and the new Ultimate Technique can't be used whenever you like, but needs kills to activate, this makes the game difficult.
Keep in mind, that's just the first level, and it isn't even the "hardest" difficulty. From my perspective, it sounds like the game isn't hard because the enemies are better than you; it sounds like the game is hard because of the design choices. I haven't played it yet though, so I could be wrong.