In all honesty the only thing I feel feels off about 3 of the bosses in this game is how much health / armour 3 of them have. Their move sets are more challenging to learn but that's good, that's actual difficult. But making the take twice as long to kill as anything in the main game is artificial difficulty. A thing I hate. Like halving your health in the defiled dungeon. It's a lame way to create a challenge.
After getting all trophies I took my NG alt to finish gehrman and killed him in about the same time it took to get
out of phase 1. And the first area is recommended lvl 65.
If you like a game being difficult via larger health pools and bigger damage output then the game has NG+ for that. If you want an easier time it has summoning.
I don't believe anyone can look at those three bosses and say they fit in naturally with the rest if the game.
I actually think all bosses accept the optional one are really awesome fights. But when it takes as long as killing any other boss just to try and learn
phase 2 it feels a tad out if place. It's basically 2 bosses back to back without a save point in between.
The videos where they take alot less time to kill are clearly over powered chars built up moths ago when they first played the game.
So yeah, those bosses move set are fine, just the health / armour is totally out of place.
For reference I started dark souls 2 last week and the first 4 bosses in that make cleric beast look insane hard by comparison.