Hardline existentialism, like that of Sartre and de Beauvoir, can be invalidating at times. It really takes an extreme stance against outside circumstances. Merleau-Ponty's existentialism takes into account mental illness as a barrier to personal choice, but still it can be invalidating. A hallmark of personality disorders is that one really is shaped by outside circumstances and finds oneself in many crappy situations, but at the same time the disorder itself lends to an irresponsible way of looking at things, like bad faith (blaming others and circumstances for bad behavior and choices). It's a hard stance to take and one needs to be absolutely ready for it. I consider myself an existentialist, but to take a hard line stance like that of Sartre can be detrimental. Humans are complex and theories and religions in general tend to be reductionistic. I have a similar take on psychoanalysis, it is reductionistic. I believe one must take an eclectic approach to things.