As I see it, there can be no guarantees about psychotherapy outcomes other than one. The person who comes to therapy regularly and develops a meaningful relationship with their therapist can be guaranteed to know themselves better. That might sound uninspired - like a waste of time and money. On the surface, that's a fair response. We know our lives better than anyone else ever will. Usually, we have some real insight into the issues that bring us to therapy. From that perspective, it's understandable that we begin therapy with the hopes that someone would teach us something new. While teaching can be helpful it doesn't create the type of change that helps us move beyond our default patterns.
The sources of our problems and the solutions to them lie below the surface. The problems we have wouldn't persist otherwise. It's inevitable that if these thing continue to stay below the surface we continue to contribute to the feedback loops that leave us in emotional turmoil.
All relationships are mirrors that show us who we are. Some of those mirrors are warped. Exaggerating and diminishing different parts of who we are. Our relationship with ourselves included. A good therapy relationship is a caring, clinically informed mirror that helps us see more clearly how our problems came to be, how we continue the feedback loop of the problem and what we can do to change it.