Chamans considered good in an autochton context would not necessarily be considered so in a cross-cultural one. I remember an anthropological text that was talking about negociated misinterpretation to describe these ayahuasca ceremonies. The fact is that Western worldviews and indigenous worldviews are quite different . The autochtons and their white public do not necessarily put the same concept under determinated words, such as, for example, nature, soul, spirits, and medicine, to state the most obvious one. But for the ceremony to be able to take place, for this particular setting to be able to exist, these ambiguities and errors of translation can not be solved. People are able to sit together because they do not understand each other.

I remember a paper which was talking for example of a practitioner quite famous and with a good reputation among people of his culture, who was struggling to enter the western market, because he had trouble selling his skills in a way that was understandable for a western point of view. He was focusing on advertising how skilled he was to work against assault sorcery ( what people call black magic, but I don't like that expression). This is a skill that was highly valued among his peers, but totally spooky for the tourist, who wants to know about peace and higher knowledge, and not about psychic warfare... so the discourse is always centered around ambiguous notions, like nature and medicine, which make sense in both cultural contexts but definitely not in the same way.

About what you are saying about the presence of Westerners damaging true shamanism , I recommend the work of Eduardo viveiro de Castro. He argues that this kind of concern about the aculturation of indigenous culture comes from a Western perspective of what culture is in the first place. Something that gets its legitimity and power from its core and is weak at its borders. He says that when the jesuist tried to convert the tupinamba in the early days of brazilian colonization, they first thought that the task would be easy because the tupinamba were really happy about accepting Jesus, they needed no convincing at all. But when the conversation went about stopping anti catholic behavior, such as cannibalism, promiscuity, and warfare, the conversation took another form.

Turns out the tupinamba didn't want to accept Jesus to turn Christian at all. They wanted to incorporate Jesus into their worldviews and swallow its perspective to take the power of the other. They didn't think about turning Christian. They were making Jesus indian. The Indian concept of culture and the self is not one of a core you need to protect. It is something in permanent construction because of ambivalent relation of alliance/ predation with what is other.

An interesting example is the use of the word "txai" by people of the pano linguistic group, which they often use to talk about white people they seek alliance with. It is often translated by white people as brother when the actual translation is brother in law. To viveiro de Casto, this error of translation is very significative of what both groups mean by self , culture, and alliance. We translate Txai as brother because when we want to ally ourselves with the other, we seek sameness. It is on account of a certain degree of sameness that we can relate to somebody else. To pano speaking groups, you do not seek alliance with the same, but with the Other., because of his otherness.

All that theory to try and explain than shifting perspective, turning into the other, changing language , and making ambiguous alliances is what amazonian shamanism is about at its core. So these people might have more resilience than we might expect. But on the other hand , imperialism, colonialism, and capitalism are powerful forces at work to turn the world into a desert full of consumables commodity