And where would a layperson find the NICE guidelines? Googling it, I think I've found them for anxiety and depression, but they suggest over five different types of therapy, so not really narrowing the field down! Especially as, until a person tries a therapy they can't tell how they'll respond – and even then if they don't respond well, they won't know if it's due to the psychologist being the wrong fit or poor at their job, or themselves.
As for 'having a clear idea of how you measure success', how exactly is one supposed to do that, in the short term? I've read that therapy can make a person feel worse before they feel better, that it can take multiple sessions to show any effect – even that it can take many sessions just to reach the core of an issue, let alone begin to work through it.
It sounds like you're expecting (depressed, anxious, traumatised etc) patients to be as qualified as the average therapist! That's not realistic. I don't have tell my dentist what treatment I need – they can tell me, and the results are immediately measurable. That's the difference. So how much money will one spend before they realise the therapy isn't working, either because of the method, or because the therapist isn't very good?
Even then, it seems like you might be keen to blame that on the individual and their 'personal issues', rather than understanding that all the factors I've mentioned can lead to cynicism with the industry.