The problem is this is a very complex question. Many people it fails for may not put the effort in, may stop applying the lessons learned after they feel better, may experience a life event that causes a relapse, may have more complex issues that remain undiagnosed or unaddressed, etc.

How many people get referred to CBT when they have more complex issues like personality disorders, severe depression, undiagnosed bipolar, etc? The IAPT services will try to work on what they can or refer you into the CMHT's. Where they try themselves their failure rates will include these patients.

Then we have the issue of studies & benchmarks and how they apply in vivo (real world). Many studies are conducted without "dirty data" (they should be really as dirty data will cause studies to fail unnecessarily) but the problem is in the real world we patients don't come in neat packages that fit to lab models. The same can easily be said of antidepressant testing as they remove anyone remotely not in a range they want to test on.

So, you get situations where success rates may be based on less complex cases. And what about the variation in the skills of therapists themselves? Studies may be "gold standard" with psychologists who are much better trained & experienced than many of the lower tied staff we may see within IAPT.

Then we have the issue of anecdotes. Given the complexity of all this it's very easy for a patient to complain something doesn't work without telling you all the details you need to determine whether they worked hard enough on their issues, had a good therapist who tackled their issues the right way, etc.

I think, like axolotl says, you don't look at it as a cure but as a treatment that may improve you. To wary of "the cure" because whilst we are understandably crying out for it, it can add unnecessary pressure to get better. I prefer to look at it as a tool in the toolbox and a matter if finding what works. Whatever ground you gain in how they help, great! It's not enough but even some small improvements are better than nothing and we keep going from there.