Most Healers can work "successfully" for several reasons.
A patient never verifies with objective means if a Healers diagnosis exists outside of the imagination. It happens as patients over-rely on sensations and feelings. The placebo effect can be painful, and imaginary diseases stress the mind and the emotional body, leading to various symptoms manifesting or some physical damage sustained. Then a person will still experience relief from the treatment and could be trapped for decades in a cycle of going to the same Doctor and paying large sums of money for never-ending treatments from never-ending virtual diseases.
A patient heals from some actual disease, and Healer would claim all the credit for this thing to happen to his effective methods and skills. They are playing the role of merciful God and forgetting that the Human Body is a living organism that constantly regenerates, repairs, and renews cells. There is a reason why all medical clinical trials go with placebo groups and use the blind testing method. (Nobody should know if they take working medicine or just a sugar pill) Many ailments can heal on their own and go away naturally as time goes by. As it is said, it does not matter how you heal the regular Flu, with over-the-counter medicine, with herbs, or with shamanic rituals; it still takes, on average, a 7-day period to recover.
Many Healers aim for this timing gap, producing treatments that have to be done daily for 3 or 6 months or more extended periods.
The placebo effect is a well-studied phenomenon. Despite this, the homeopathy business is worth billions of dollars; all it does is sell sugar cubes and pills with mumbo-jumbo names. Looking at the scale of the fraud, you can see the degree of the mental sickness human society experiences.