Black Obsidian is one of the crystals that are so fast in being effective. I don't think there is 'hate' about it, and I think I understand what you mean.
There is a problem here, and please keep reading as I take a long way to explain until my point hits.
For example, let's say you are in a very bad moment of your life, no energy, no mood, no money...apathy.... Now, you are using black obsidian. what happens is that you will see some of the negativity and perception of what's going on, being 'reduced'.
However, black obsidian, you cannot hide anything from it, it will show you the exact cause/effect up to the root of your problems. it does work with your shadow side, and in my opinion is your duty to understand to learn how to do that; nuumite stone is also related to shadow side, but let's get back to my point. You will realize/see/think about your actions, the root of your problems, your shadow self (or how some people may say, the real you, but the real is ying-yang, not just what is 'bad').
AND because you are starting to realize all these things, and you see what you could have done, and what you NEED to do... it goes a bit in a conflict when you do not have will, feel down, no money....and this can send you in feelings of regrets which may be amplified by the black obsidian. it takes a lot of energy to stand up and fix what you need.
But there is more to this, than just what I said. I am pretty sure there are things I may not even be aware of. I agree, there is something a bit 'spooky' about it. Some time ago... while I was doing an experiment* created by Albert H. (no more details here)., I was checking my crystals and I was able to see their energies like no other book can ever explain, simply put, words would make no justice to describe my experience. With that being said, when I reached to my black obsidian, it was not like any other crystal, where I could gently analyze their energies and such. No, the black obsidian, my focus went straight in, it felt like in those movies where the sound reduces with 90% and you zoom in slowly.... Is hard to describe it, but as I was looking in its reflection, a shadow quickly came from the ceiling, with the speed of a 'thunder' so to speak, and went straight in my obsidian sphere... I quite of freaked out but not really, I quickly stood up and put my panic on pause, and went to another room where a friend of mine was laying on the couch...This was during the night too.. glad I wasn't alone. I turned on all the lights in the house while saying "nope nope :) ok, move on now, what's in the fridge~". Gave the sphere to him to check it out. No idea what happened... but I did had an encounter with an entity 3 years ago, and it was definitely not like this! As 3 years ago that moment was one of the most beautiful moment in my life and I will remember it until I will die and after that hopefully =)