Just finished my Hard playthrough. Pretty much everything I said before still applies, but what really hurts this game is the complete lack of variety. On paper, there are lots of different enemies, but damn near all of them are humanoids, so most of the battles play out the same with some minor variations. There are a few fiend-type enemies, but you're fighting humans probably 90% of the time so they don't make much of a difference. And even that wouldn't be so bad if you had more than one god damn weapon. I'm already starting to get tired of the repetition, which is pretty ridiculous because the previous games can stay fun for hundreds of hours.
The game is okay though. Basically, it's a decent action game, but a bad Ninja Gaiden game. I will say that I liked it more than I thought I would, but my expectations for this game couldn't have been any lower, so that's not really saying much at all. Gonna take a little break and start my Master Ninja playthrough next.
And the Hard difficulty achievement doesn't stack with the Normal and Easy achievements, but apparently you get the Easy achievement for clearing the game on Normal. Why. (I think you can reload the last checkpoint and lower the difficulty to get the achievements anyway, I just thought that was stupid enough to point out.)