I’m one of those people who only have second-hand knowledge of existentialist writing (though I did read Camus’s ‘Myth of Sisyphus’ – without really ‘getting’ it). But what I learned seemed to me to be positive ideas. I especially appreciated existentialism’s emphasis on personal responsibility/autonomy and its inherent humanism.
But once I read up on neuroscience and also adopted a naturalist/physicalist view of existence, I started to question whether the basic tenets of existentialism were correct. If our minds i.e. our ‘selves’ ultimately derived from physical processes in the brain, doesn’t this kind of determinism refute the so-called freedom Sartre said we were condemned to?
Furthermore, the disciplines of psychology, sociology and anthropology have advanced enough to give strong credence to the idea that our characters and choices are to a significant extent affected and constrained by factors beyond our conscious will. Add to that the current debate involving Sam Harris, Daniel Dennett, Russell Blackford and others on whether such a thing as ‘free will’ even exists, and suddenly existentialism appears to lose credibility.
I wrote about this awhile ago. Feel free to persuade me of the error of my arguments. 
