I think that one particular point that can be made in arguing against Existentialism is that it clings to the similar, false sense of reasoning that it claims of other modes of though. Existentialism is certain in its belief that there is nothing outside of the individual. It clings to this belief with absolute certainty. A point can be made that Existentialism offers little in way of proof or evidence about this. It simply asserts it as truth. Its rejection of totality or transcendent notions of the good is only evident because of a perceived understanding. Like its criticisms of transcendence as "bad faith," it does not offer anything of verifiable means. To a great extent, Existentialism does not want to do so because in offering such a grounding, it would succumb to an embrace of totality, something that it does not wish to do. In not offering anything other than assertion, it becomes victim to the same thing it criticizes.
I think that another significant point to make in the critique of Existentialism is that it is reductive. Its reductive thought makes clear the idea that there is nothing else out there. Yet, in asserting that there is nothing out there and firmly asserting it with a sense of totalizing certainty, I think that Existentialism might be asserting a totalizing notion. This would be an example of "bad faith," a situation in which the individual takes sanctuary in order to avoid facing the complex and intricate notion of reality. It is reductive in making the case that there is "nothing" out there. It is far more complex to embrace the potential reality that there might be some force of transcendence with as much certainty in how there might not be one. If the Existentialist is concerned with not giving the individual a way out through "bad faith," being able to hedge all bets of totality (whether it exists or not) would facilitate this. In doing so, Existentialism might yet again find itself guilty of the same that it critiques in other theories of being in the world.