There are two main problems:
The first is that Primal therapy has never been really, truly, properly evaluated.
The second is that NO psychotherapy has ever been really, truly, properly evaluated.
I don't say that to be flippant. A lot of claims, for instance, are made that Cognitive Therapy is the One True Therapy because several studies have proven it to be as effective as medication. And most of the studies have been conducted or underwritten by insurance companies, since Cognitive Therapy is usually a brief therapy, ergo, requires less payment. The quality of these studies tend to be questionable, the Peers doing their Review often have financial interests in the results, and the precision of the statistics is usually low.
And then there's the human and phenomenological side of the studies -- virtually absent. I've had a lot of congitive therapy (there goes the theory about it being a "brief" therapy) and all it really did was teach me to mask my emotions. Now, when I get depressed, I no longer get sad, weepy, or painfully self-pitying; instead, I get much more "physically" depressed, which for me includes migraines, severe IBS, and a number of other symptoms. The depression is much more "retarded", or energy-draining. While the cognitive techniques work well to help me deal with day-to-day problems, the depressive illness remains in place.
And drugs? Not an option. The only antidepressant I don't have severe reactions to is an over-the-counter supplement called S-AME.
Art Janov was probably the first psychologist to subject his therapy and his patients to a wide range of polygraphy and vital-sign testing. Jung had been interested in skin galvanometry for a time (the forerunner of the E-Meter) and several studies were made documenting symptom reduction with Psychoanalysis, but there had been no systematic, long-term study of the body's measurable reactions until Janov. And yet, until other therapies and control groups have been studied in the same manner, all we can really say is that Primal therapy achieves an interesting set of outcomes that are desirable (such as reduction of blood pressure) but have little or no context.
I doubt that the kind of studies required will be funded any time soon. Primal and other "dynamic" therapies are more expensive since they are not brief. And they're not the purely technicianly pursuits that most brief cognitive therapy is, so they tread on the turf of Psychiatry.
The Primal Institute under Vivian Janov's direction has begun to offer a briefer Primal therapy. It's still a long-term pursuit, and the brevity is mainly related to the intensive period (8 vs 21 days). It's cheaper from the point of view of insurance companies (which is helpful in financing therapy), but not fundamentally different.
Unfortunately, it's going to take some time before psychotherapies can be properly evaluated across the board, and subjective response is going to be the "gold standard" of therapy. Until then, proponants of therapies are going to promote their own therapy, occasionally coming close to the efforts put into Primal therapy, and the opposition will mainly publish CSICOP-style "debunkings" which focus on select features of what they THINK the therapy is about, mainly to get a laugh or a shudder of disdain.
The Answer will be a long time coming, I'm afraid. Until then, we'll have to do what we've found works best for us. Which, for a small number of us, is Primal therapy.