Oh interesting, I hadn't thought of this from a therapy point of view. Any time you mix spirituality and treatment my mind goes immediately to scam, which is what I assumed this was.
Any time there is treatment, there is spirituality. It's just that western medical approaches pretend there is none, whilst a lot of other approaches pretend there is more than there is.
The New Yorker wrote a good piece on the use mushrooms and hallucinogens (which are similar to ayahuasca) having the ability to improve mental illness, depression, addiction, etc. They are testing this is troopers who return from battle http://www.newyorker.com/magazine/2015/02/09/trip-treatment
It doesn't sound like your definition of spirituality and my definition match at all. IMO, spirituality is understanding the significance of you, all of us, and the universe; it's empowerment of self, and understanding that we're all essentially "gods", able to change reality as long as we put our mind, and actions to it.
The shaman by him/herself won't do anything. Neither would the Ayahuasca on it's own. It's having the right combination of both (setting), and your intention (set) that leads to an amazing experience.
The funny thing is, the experience itself is not the most important thing.. it's what you do after the ceremony with your life, and how you change your lifestyle to make you happy.
Obviously, if the shaman isn't giving you the real tea, mixes stuff in, doesn't take care of you, and such, I'd call it a straight up scam.