A true control seems impossible for any "breathwork" study; anyone doing regimented breathing knows that they are doing something. And placebos work, whether you believe in them or not (say the studies ...)
If you can't show that a specific breathing exercise is somehow better than an essentially random breathing exercise, then it doesn't matter what kind of breathing exercises you do.
If you can't show that a breathing exercise is better than e.g. same time spent on stretching, then there's not much point in doing (separate) breathing exercises at all.
