"Scientism" tends to be used as a pejorative rather than a self-designation, we can probably agree on that. But the question is whether it can be well defined and has valid application. I think it can and it does. The original question was whether scientism is "nothing but" an insult. I would agree it's mostly used as an insult, but that it also has substance.
Scientism is the view that only science or the scientific method can deliver real knowledge. Other kinds of knowledge are either either collapsed into science or else they are not classed as real knowledge. Dawkins tends to do this when he disparages philosophy. And Harris does this when he claims ethics can be reduced to a branch of science. Dennett does this when he says consciousness can be reduced to physical properties. It's an extreme form of materialist reductionism. It is a fundamentalist position held by people who claim to oppose fundamentalism.