My repeated experiences (over 30 years) at numerous training sites, thousands of hours in the classroom as student (!!) and in hundreds of hours of being supervised in psychotherapy, I rarely hear this 'complexity' talked about in any meaningful way.Listen to people firmly entrenched in their theories. Do you ever hear cogent questions or statements about what is 'unknown'. You simply don't hear people question themselves in any kind of public forum. We've (psychotherapists) published books that would stack higher than Mount Everest, but see how many sentences you can find that pose fundamental questions about what we do not understand about psychotherapy.
Such questions were never posed in any of the above case conferences. To an outsider reading our textbooks or reading this forum there would be the strong impression that we have identified all the basic fundamentals of how to change the human psyche. The only problem is to how to fine tune this or that, and that while all our approaches may be different, that we all still arrive at the same place in terms of understanding and helping a patient or client to change. And yet, ask any one of us if we are resistant to change, or hidebound in a particular way of thinking and we would all protest, "of course, I'm not".