I don't think anyone would suggest that there should be perfectly consistent outcomes, rather, on the whole that outcomes should be positive. We know there will be poor performers.

The study is suggesting that 'on the whole', there are negative outcomes, implying that 'most psychotherapists' are just bad at what they do, or more likely, there's a more fundamental issue with the nature of psychotherapy itself.

All these things have something in common.

Martial arts is definitely an area where I don't give it much stock because a lot of it seems rather iffy. If that's where therapy is, it'd be good to know.