Part of the problem is idolatry. Another aspect of the problem is the claim that their idolatry is in accord with the Law of Moses. That is a kind of fraud.
I did a great deal of study on this, which is sad- because if I had been smart, I simply would have accepted what the Gra said, and not have wasted years. But the human ruin that that movement leaves behind it ought to give any decent person pause. If it leaves behind it a trail of human trash, then you know it is a cult.
I am very well aware of their doctrines and teachings. Have nothing to do with them at all. Do not go to their place on Shabat and don't read their books. It is pure Sitra Achra.
This is why understanding the cultural assumptions tells you a whole lot about the trajectory a culture is on. It’s also important to realize some people can change a culture for generations by either positive or negative actions, given their place.
But some ask, why did the Gra put a Cherem [excommunication] on the whole movement when not everyone was bad? The answer is that poison got mixed up into it. What if I tell you, " Here’s a box of candy. One of them is poisoned with Ricin, but don’t mind that. Enjoy." Would you eat any?
The laws of Cherem are strict. One that ignores them is also under Cherem. There are difference between types of Cherem and Niduy, but in case of fact the cherem is the most strict. And it does mean one can't sit within four yards of the person nor learn Torah from them.
If others ignore these laws that does not make them invalid.
If one ignores the cherem it is guaranteed he will become infected
It is a kind of deflecting people's interest in fear of God and to redirect it into something that destroys their sanity. While fear of God is a good thing and ought to be pursued by means of learning books of Musar [Mediaeval Ethics], still this desire to be right with God can be hijacked.
To a large degree this happened with me. I was at the Mir in NY and often instead of learning Musar I would learn books from that cult thinking that qualified as Musar.
It took me a long time until I starting noticing the poison contained in them. I needed experience with that cults and also to see the books from the Shatz cult to see from where many of the doctrines were coming from. It would have been better if the Litvish Gedolim [the great Roshei Yeshiva of Lithuanian Yeshivas] had been aware of this from the beginning and warned others, instead of my having to make my way through that filth.