I don't think I'd compare Bloodborne to Ninja Gaiden when there are examples like other Souls games or Nioh to compare with. I think the upcoming Code Vein will be another one.
I think something people misunderstand about FROM games is that they aren't hard because they're cheap. They're hard because they're obtuse and that can be good or bad.
I'm not sure where I'd draw the line between cheap and obtuse when it comes to a boss' attack power/speed/hitboxes, but from my experience the main thing that stands out for me regarding bosses in these type of games (NiOh included) is not that they have an unusual amount of techniques you need to learn how to deal with. But rather that once you get hit, it hurts a lot more.
Though there was one mechanic I particularly disliked in Bloodborne, that lead to increasing the amount of time it took me to take down bosses. But it wasn't related to the boss itself.
Usually in games when I lose to a boss, I want to just go back and try again.
But in Bloodborne, once I ran out of Blood Vials, I had to farm the two nearest enemies next to my save point for more.
I found this to be a break in immersion, and a demotivating element. It lead to me turning off the game in instances where I normally would have ran back to the boss a few times instead.
In NiOh this was not an issue because even if you spend all your health potions, the game still gives you a minimum amount of free potions (which can increase if you upgrade the skill) when you revive. It's not a maximum of 9, but enough to give you a proper chance at getting to that phase of the boss where it starts using the moves you're having trouble with. Or even defeating it. But starting with 0 just doesn't feel right when the game is designed to let you carry 9.