Let's start with Jennifer's premiss: Tarot cards have symbols and thus they cause people to create interpretations based on their interpretation of the symbolism... Oh, and a psychologist/therapist can interpret those interpretations to help a person somehow with therapy based on the interpretations of interpretations.
Now we're talking! That sounds like pseudoscience!
Sure, I like the core that they are just pictures, but Jennifer may have a problem with basic science. Oh yeah, this is psychology... There is not a lot of science in psychology.
Anyway, here is the flaw: You can't tie any response to a card to root causes. It is just sort of impossible. Worse still, the interpretation of the interpretation depends on the interpreting observers history and assumptions.
Let's take the simple case, say the patient plops down the Death card... The patient's interpretation is based on their life experience and pre-wired assumptions that were built on that experience. Their reaction will be based on too many to count, let alone trace to a single root cause for the interpretation. Their reading might be that they see change, death, loss, or even feel that the therapist is indoctrinating them into the devil's science. Those interpretations are just what comes out, there is no way for sure to understand why.
On the other side, the interpreter, say a therapist, has their own assumptions pre-wired. Their life experience does the same thing to both their focus of observations and their interpretations. For example, if the patient is seeing loss, the therapist could believe there was a recent loss in the patient's life or assume the loss was related to childhood trauma. Or, as would be rather obvious, the idea of loss is just what they learned was the interpretation of the card from a friend, book, movie, Oprah, or combinations.
The bottom line though is that there is no way to run an experiment. You know, that silly nerdy stuff called the Scientific Method. This is a problem with most psychology. Not that I place psychology into pseudoscience, but it will always be on the edge of full blown testable and provable science. There is just no way to get a repeatable and independently verifiable result form human brains. Too messy! If they weren't messy we wouldn't have all those religions or more than one political party.
Sure this could be a tool. But as a tool it is one of the worst. With a hammer and nail you can fairly accurately drive a nail into a piece of wood(given a little training of course). There is no way that Tarot could be used to any level of accuracy. It is more like a wet noodle with the nail driving itself from belief –poor metaphor, but I am a result of all my experience to this moment in time.
I guess the danger is in just one word used: Interpretation. The moment we 'interpret' we are on the slippery slope of pseudoscience and the danger of conclusions based on belief rather than reality.
That's the lesson. Want to be a great pseudoscientist, be an interpreter of interpretations.