Which form of therapy is most likely to be criticized for offering interpretations that cannot be proven or disproven? a?

I think your question is missing options, but I can give an example for a therapy that is not known to be supported by scientific research, but which is still often used because it was not disproved: Sigmund Freud's psychoanalysis. In Psychoanalysis people try to understand their unconscious thoughts to help them understand why and how they feel.