I think maybe a better question is not if markets are fully efficient, but if markets are more efficient than the alternatives. I read the Cassidy book years ago, so I might be misremembering, but I don't think that was something he spent much time discussing. I don’t think many would argue that markets can't fail and aren't fully efficient - he spent the entire book demonstrating that.

The idea that regulators have the ability to design, implement, and maintain laws that correct or protect from market failures without producing unintended consequences, perverse incentives, significant costs, or market distortions more severe than the original market failure was not sufficiently addressed, from what I remember. Despite their problems, markets tend to be dynamic, adaptive, and innovative, while regulations tend to be rigid, slow, and reactionary. I don't know what the answer to market failure is, but I found the book unsatisfactory in that regard.