Reddit mentions of Bergin and Garfield's Handbook of Psychotherapy and Behavior Change

Three references for you:

Smith, Mary Lee; Glass, Gene V; & Miller, Thomas I. (1980). The Benefits of Psychotherapy. Baltimore, MD: Johns Hopkins Univ. Press.

The Great Psychotherapy Debate

Handbook of Psychotherapy and Behavior Change

I am not an expert on this question. But Bergin and Garfield's Handbook of psychotherapy and behavior change, 6th Ed. suggests humanistic-experiential and psychodynamic psychotherapies, as well as some behavior therapies, have much more research in this realm. I suspect it's partly because they can never win the fight on the realm of number of studies establishing their efficacy (every study that therapy X does to establish that it is at least as effective as CBT contributes to the CBT literature, so therapy X will always be behind). Also it's hard to get funding for that anymore (since we already have a "well-established" therapy). Furthermore, client-centered approaches lead to client-centered research, so a bulk of their research is about how clients change. Similarly, psychodynamic therapies have wonderful deeply examined case studies that facilitate understanding at a sequential causal level how that particular person changed. Yes that's not generalizable to the population at large, but when your n=1 studies are in the hundreds (probably thousands), you have learned a lot about how people change.