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.
I think that line is different for every person which is why peoples' reactions to the way these games are designed vary greatly. And for sure the bosses hit a lot harder than in a conventional action game. But to counter that the arenas are typically larger and the design of the dodge/roll mechanic is such that if you time it right you won't take any damage at all.
Where other action games may put an emphasis on using the right item or keeping distance or even just absorbing damage while finding a way to deal more in return (like being able to cancel an attack to dodge), the Souls-like games focus on a different way of dealing with bosses so it's on the player to learn those rules and apply them. And I think that's why some people think the games are poorly designed; because the rules the game imposes are different than what we're accustomed to. And I had that same reaction. I was stuck on Father Gascoigne for over 3 years. I definitely get the frustration with coming to terms with the game's rules lol. I farmed that path hundreds of times. I'd go back to the game every couple of months, try and die for 3 hours straight and then give up for another few months. The games can be extremely tedious and I think each person will receive that aspect differently as well.
I haven't played NiOh but from what you're describing it looks to be a mix of Souls-like design and conventional design with a greater focus on the combat system providing options.
I also totally agree that I wouldn't compare NG to Bloodborne/Souls. One is difficult because of the dexterity and focus and reaction times required while the other brings a lot of challenge from coming to terms with mechanics, items, build, and world design.