According to my own experience, yoga is mental and physical involved. I used to take both yoga and Zumba in gym. For a level of beginner to intermidium, one hour yoga will cause me feel tired for whole week. And I take Zumba which is either 45 minutes or 1 hour. Usually my t shirt will be like water soaped after it, I feel ok for 3 zumba classes each week, plus one ballet and 2 mornden dance which actually very energy consuming. I keep energetic and fit all the time by those exercises.

I don't think I should take yoga , so I quit year ago. But I don't feel surprised there are negative comments on it now. People follow the trend , but they should know if it's suitable for them at first.