From this NY Times article, Does Yoga Build Strength? The question in the title is asked and their answer is:
“In general, the few available experiments involving yoga suggest that it leads to measurable but limited and patchy strength gains.”
Is yoga sufficient strength exercise for optimal health? The question is a straw man. In the yoga classes I have attended increasing flexibility and mobility, not strength, was the central purpose.
The six factors of fitness are strength, flexibility, body leanness, resistance to injury and illness, enhanced bone density, and increased cardiovascular efficiency. The degree to which yoga helped me improve strength or the other factors of fitness, I don’t know or care; I simply felt better afterwards. I tell my clients to do something they enjoy and they will be more likely to stick to it. For many, that is yoga. I tell them to strength train too.