Doubting Makes Sense at Times

Many times I have read or listened to some unjustified or outlandish claims about the value or benefits of yoga or qigong.  Some claim one can obtain amazing, otherworldly, special or magical powers (siddhis) from the practice of these arts.  Some claim that all kinds of ailments, diseases, and illnesses, both physical and mental, can be cured or overcome with diligent "correct" practice. These arts are also connected with religious viewpoints (e.g., Taoist, Hindu, Tantric, Buddhist) that also make some questionable claims about the benefits of various spiritual practices. Some extraordinary anecdotes are very suspicious and should be doubted. 
There are definitely benefits from the correct and sensible practice of various mind-body arts and spiritual practices; and, there are definitely limits to these benefits.  Keep an open mind, be somewhat skeptical, courteously question, and test claims against reality.  Look at the other side of the coin.  Don't believe all that you hear or read.  Weight anecdotal claims against common sense, averages, and personal differences.  Be careful of an over reliance on magic.  Avoid being gullible.  Respectfully listen to "authorities," but do not always accept their recommendations and claims on blind faith.  Watch out for fakes, phonies, and hucksters.  Keep a clear understanding of the limitations of your own body and mind and how certain practices might not be beneficial to you and even detrimental to your health and well being.  Do some research, listen to other experts, read widely, get a second or third opinion, and think clearly and wisely. 
Willpower and faith have a place in beneficial body-mind practices, but cannot force or control the flow of the Tao just as you desire.  Accept some defeat, loss, failure, errors, misinformation, and falsehood as part of the reality of the life and mind.  Uncertainty and chaos also share in the spotlight on the stage of life. 
Two books that I have read might help you in keeping a more level perspective on these topics with respect to the practice of Yoga:

The Science of Yoga: The Risks and the Rewards  By William J. Broad.  New York, Simon and Schuster, 2012.  Index, bibliography, notes, 298 pages.  ISBN;  9781451641424.  VSCL. 
The Hindus: An Alternative History  By Wendy Doniger.  New York, Penguin Books, 2009.  Index, bibliography, notes, 779 pages.  ISBN: 9780143116691.  VSCL.