I played a lot of Ninja Gaiden Sigma in middle school 6 years ago, I beat it on very hard and got past Alma on Master ninja, but I got stuck on some part after that and quit. A few months ago I returned to it and I got past that part easily, but got stuck on another part, one where you need to beat like 20 or 30 of the hardest enemies in the game in a row(with three on the screen at once) in order to get a health upgrade item, and I got sick of getting my ass kicked, might go back to it and just skip that part. They keep doing this explosion attack that obscures the screen for a few seconds so you can't see what's going on, it's really annoying.
I guess it's worth throwing out there that I hated the Sigma version. More loading screens than Black and even the framerate didn't seem as good, then the included Rachel parts were -awful-, completely hated playing as her and being forced into it. The original game had tons of extra content as is anyways.
Man I love this game. I just bought again a few weeks ago for a couple bucks. This is probably the hardest game I have beaten but wasn't so bad to make me want to quit. I think the comparison to metroidvania is pretty accurate though I had never considered it before. Alma was tough the first time & I remember getting killed by the tank boss dozens of times, I think I actually quit for a week or something because of that. This is one of the best video games I have ever played.