The idea of non-duality as it is usually presented is used as a tool to deconstruct things. It is used to question feelings of identification with some things and separation with other things. It can be a good thing to study these things, but it can also be used to undermine criticism. Like if someone says "I don't like that the guru does this or that", then people will say "this is your ego defending itself". Trying to make one with the guru is literally part of the buddhist teachings and part of the practice of guru yoga in tibetan buddhism. Making one with the guru is equated with dropping resistance and dropping criticism. This is part of the "samaya" vow, for example, vowing to never criticize the guru or the teachings.
In the cult where I was, non-duality was used to undermine the concept of right and wrong, and being beyond right and wrong was regarded as a very enlightened quality. At the level I was it was still quite mild, but at a higher level, in different circles it had been used to condone physical violence from the guru, like punching people, kicking them while they were on the ground, torturing animals. There were several testimonies from different people, and the current guru had to leave the country but still has followers. I have met people who told me they received punches to the face from the previous guru and took it as a sign of "compassionate anger" meant to help them on the way to enlightenment.
The followers who stayed there are mainly separated into two categories. Those who didn't experience the manipulation first hand during their first years, and don't belive the acts of violence were possible, or that they were just mistakes because there are totally incompatible with their nice experience during their first years in the organisation. And there is the group of those who think they are advanced enough that they can see the enlightenment through these acts. The problem with the second group is that whatever criticism they receive, they think that those who criticize simply don't have the same realization as them and this is why they don't understand the teachings "properly" like them. This is why it's a trap, because each person makes their own version of what the path of enlightenment is, and obviously it's where they are and not others, and they use this just to validate their own position even more.
You could think it's an isolated cult, but take one guru like Sogyal Rinpoche, he was seen punching a nun in the stomach during a public representation, later women accused him of sexual assault, in a close circle he was beating people, shouting at them, making women wipe his ass (I mean literally). You could think anybody with common sense would regard him as a fraud, and yet after his death he was incensed by other very famous official masters for being very enlightened. To me this is one possible result of that kind of deconstruction when it is used to manipulate.
In the cult where I was, the most serious signs of manipulation I started to notice was when they asked us to meditate on our resistance against what the guru could be saying or doing (that was before the testimonies about him came out and most people were not yet aware of what he was doing). They made us meditate on our criticism and resistance, just to make it evaporate, to dissolve it as we usually do when we meditate. A lot of people didn't see any problem with that because they had already been regarding anger as something only negative that impedes proper judgement. But when their anger was gone they dropped the criticism and they followed him. If the answer to a question bothers you, just deconstruct the question itself on the ground it is made of concepts and is based on duality.
I don't think buddhism and meditation are equated with manipulation, but it is a very good toolbox for those who want to manipulate others or themselves. Also the fact that the buddhist concepts are often blurry helps to defend them as people often slightly adjust the definition to their needs and to the context and regard others are not advanced enough on the path if they disagree with that definition.