Is yoga too dangerous for a weekly class?

This Sunday, the Times will publish a radical article, How Yoga Can Wreck Your Body. It largely features New York teacher Glen Black, who is very concerned about people doing advanced yoga poses, including headstand, and creating long-term health risks.

Black has come to believe that “the vast majority of people” should give up yoga altogether. It’s simply too likely to cause harm.

Not just students but celebrated teachers too, Black said, injure themselves in droves because most have underlying physical weaknesses or problems that make serious injury all but inevitable. Instead of doing yoga, “they need to be doing a specific range of motions for articulation, for organ condition,” he said, to strengthen weak parts of the body. “Yoga is for people in good physical condition. Or it can be used therapeutically. It’s controversial to say, but it really shouldn’t be used for a general class.”

Woah! What?

Mostly the article features some pretty strange cases, like:

• A guy who "would sit upright on his heels in a kneeling position known as vajrasana for hours a day, chanting for world peace. Soon he was experiencing difficulty walking, running and climbing stairs." Diagnosis: He deadened a branch of the sciatic nerve with all that sitting.
• A woman was doing uhrdva danurasana prep (head balanced on the floor before pushing up) and suffered "no sensation on the right side of her body; her left arm and leg responded poorly to her commands. Her eyes kept glancing involuntarily to the left. And the left side of her face showed a contracted pupil, a drooping upper eyelid and a rising lower lid — a cluster of symptoms known as Horner’s syndrome."
• A regular yoga practitioner suffered bruises up and down his neck and neck trauma, apparently from following Iyengar's prescription to "stretch the neck maximally against the floor."

On a more mundane level, Dr. Roger Cole, a Iyengar yoga teacher, emphasizes using blankets in shoulder stand (dangers of doing an unmodified shoulder stand: muscle strains, overstretched ligaments and cervical-disk injuries) and Timothy McCall, a physician who is the medical editor of Yoga Journal, "called the headstand too dangerous for general yoga classes" (risks: thoracic outlet syndrome, degenerative arthritis of the cervical spine, and retinal tears). Do you think the fears are overblown? Or do you think yoga teachers are not adequately educated in the risks and appropriate modifications? If you're a teacher, does this article give you pause?