I didn't think Ninja Gaiden Black was too unfair. Ninja Gaiden II got a little bit silly though, every boss you'd fought hard against on the way up, and triumphed over after a long struggle came back to haunt you. Except the next time you fight to boss, there are two of them.
At that point, I felt as though the game had abandoned all sense and was just taking the piss. I still fought on though. For the most part in Black, your demise could be attributed to your own lack of skill, but II really stacked the cards against you and wasn't always playing fair.
The less said about the abysmal NG3, the better. I'd love to see another NG game, with the furious violence and level design of Ninja Gaiden II with a little more of Black's delicate nuance and balance thrown in.