Wikipedia has an article on scientism which contains a lot of information, but also shows that the word "scientism" is used in a wide range of ways, usually pejorative.
Is there a phenomenon that could be called "scientism" and which deserves to be denounced? I suppose there is. I suppose there are people who think the humanities are worthless, or that public policy favours starving arts faculties of funds, or perhaps that we could somehow understand, let's say Macbeth, without developing any sensitivity to Shakespeare's language ... perhaps by applying the methods distinctive of science (though how you would use controlled experiments, for example, to interpret Macbeth is far from clear). The idea that science (defined narrowly in contradistinction to humanistic forms of inquiry) could answer every question would, in my view at least, be untenable. I don't see how science, narrowly defined, can tell you how sympathetic you should be to Macbeth when he learns of his wife's death and replies, "She should have died hereafter." The distinctive techniques of narrowly-defined science are not going to tell a literary scholar, an actor, or a director how that line should be spoken.
If "scientism" refers to some of the more extreme or loony viewpoints mentioned in the previous paragraph, then I think it's a bad thing. I don't think we should be closing faculties of arts or humanities, or that distinctively scientific techniques are much use in understanding or staging Macbeth, or that studying great literature is useless anyway, or anything remotely in this ballpark. But then again, I don't see many other people expressing those sentiments either. Once again, someone may think these things, but if I see Richard Dawkins, for example, accused of thinking these things I'm not going to be too impressed.
The thing is, if you're going to denounce someone for "scientism" or complain that her ideas lead to "scientism", or are somehow reliant on "scientism" - and if this is meant to be a serious criticism - you must be using the word "scientism" in a sense that denotes something horrible or foolish or otherwise worthy of denunciation. It's no use denouncing someone for "scientism" and then, when called on it, explain that you were using the word in some other, more technical, non-pejorative sense (perhaps that the person takes a logical empiricist approach to philosophy). That's equivocation. It's cheating to apply the word in some non-pejorative sense that you secretly have in mind while at the very same time trying to get the pejorative connotations of other senses of the word.
A word like "scientism" lends itself too readily to this kind of argumentative cheating. So much so that I think that intellectually honest people should stop using the word; and, frankly, when I see people using it in current debates I am automatically suspicious of their intellectual honesty. I'm pretty sure I'm not the only one.