Hal Arkowitz and Scott Lilienfeld detail the “Dodo Bird verdict,” which refers "to the claim that all therapies are equivalent in their effects." The reason why:
One explanation for the Dodo Bird effect is that virtually all types of psychotherapy share certain core features. In a classic 1961 book the late psychiatrist Jerome Frank of the Johns Hopkins University argued that all effective therapies consist of clearly prescribed roles for healer and client. They present clients with a plausible theoretical rationale and provide them with specific therapeutic rituals, he wrote. They also take place in a setting, usually a comfortable office, associated with the alleviation of distress. Later writers elaborated on Frank's thinking, contending that effective therapies require empathy on the part of the clinician, close rapport between practitioner and client, and shared therapeutic goals.
They do qualify this assertion to some extent – for certain specific types of mental health problems, particular methods do seem to matter. And, they warn, "if a clinician espouses an approach outside the scientific mainstream … you should not assume that this treatment will be as helpful as others."