I can say they are doing their job.
If someone wants to develop clairvoyance and asks me for advice, whether to go with shamanism or initiation suitable for the man of today, I wont give him advice but simply describe the differences and let them decide for themselves.
The modern path passes through self-development. Actually the self-development is primary and clairvoyance is only a consequence, a "side-effect".
These verses refer to actual spiritual perception. The single eye is the balancing of the mirrors of the head soul organ - the nondual mind state. Then we see clearly. In order for our body to be full of light we must drive out all darkness - these are the dark spirits that are hiding. When the eye is not single - we are bouncing between the mirrors of the head and we are easily confused.Luke 11
34 The light of the body is the eye: therefore when thine eye is single, thy whole body also is full of light; but when thine eye is evil, thy body also is full of darkness.
35 Take heed therefore that the light which is in thee be not darkness.
36 If thy whole body therefore be full of light, having no part dark, the whole shall be full of light, as when the bright shining of a candle doth give thee light.
In this sense, when we work to purify the mind and the heart, when we want to put our life in service for a higher purpose, we attain to seership even if we are not looking for it. Everyone is a seer by default. If we don't see it's because there's too much darkness and confusion. We are not seeing because it would be too traumatizing to see what is there to be seen. When these are dealt with, we begin to see. We first see the spirits within ourselves, then we can see them in others.
If we train to become clairvoyant by neglecting the work on self-perfection, we can still achieve something (even if the mechanism of actual perception is slightly different, but that's a whole other lecture). But if we haven't dared to lift the spirits from our own being, if we can't see clearly in ourselves, how can we be certain that we see clearly in others? If the spirits in us manipulate what we see, we have no means to know that because we don't see our own spirits and how they manipulate us.
I'm not speaking against the shamans. As said - they are doing what they are supposed to. It's just an impartial presentation of facts for anyone finding himself on a crossroad.