How would you rate the hateful entity's difficulty compared to the other NiM bosses?
In all honesty, it depends on what standpoint you are looking at for this fight in comparison to others. From my own personal healing standpoint, I felt that the Dread Guards in their original release (pre-enrage nerf, pre-damage/health nerf) was the toughest fight in all the 2.0 Nightmare content and beyond. That being said however, this fight is hard to heal, and sadly it is not because the fight itself is "challenging," but rather just stupid with all of the RNG elements thrown in.
People got stuck in walls and glitched through the world, Dread Touch threw players in random directions, cleansing for the Deathmark fell at just the wrong time when you would be stunned - a lot of elements played into making this fight hard.
Still, none of this is to say the fight is easy without these elements, but rather it just isn't that well designed. It is still a very strict DPS check for all the 75 gear we had, and from the video you can clearly see tanking the fight was no walk in the park either. Personally, it comes down to taste.