I've distinguished two strands in the use of 'scientism'. On the one hand, there's what I've called 'the mid-20th-Century' use (post 9), or the 'cold-war use' (post 11), which channels apprehensions about left-wing political agendas, especially Marxism. In post 11, I conjectured that this deployment of the term echoes some of the concerns behind the earliest uses of the French 'scientisme'. In both cases, 'scientism(e)' gave voice to dark forebodings about radical, 'socialist' agendas whose aim was to jettison much of tradition in a drastic redesign of society. Such radical programs were linked to science by their putative roots in the new social sciences.
Let's coin the label social-scientism for the kind of scientism that was being attacked in these uses. Who was supposed to be guilty of social-scientism? While cold-war fulminations targeted Marxism, the earlier French uses of 'scientisme' were more often motivated by opposition to the visions of Saint-Simon and Comte (and their followers). My understanding of the earlier French concerns is suggested by Richard G. Olson's work. In his Science and Scientism in Nineteenth-Century Europe, Olson writes:
The Saint-Simonians unquestionably understood themselves as scientists, developing a science that comprehended the events of the social as well as of the natural world. Moreover, nearly thirty years before Friedrich Engels appropriated the term 'scientific socialism' for the doctrines that he and Karl Marx had developed, Karl Grün (1817-87) had applied the term to Saint-Simonianism (Olson, Science and Scientism in Nineteenth-Century Europe, [Urbana, IL: the University of Illinois Press, 2008], p. 59).In both the cold-war and earlier French castigations, social-scientism was reviled for its supposed reduction of persons to mere cogs in a machine, social bits and pieces susceptible of the same sort of understanding to be had in the physical sciences. To be sure, proponents of social-scientism might abjure attempts to reduce special sciences to physics, denying that the subject matter of the social sciences (or of any science other than physics) could be reduced to that of any more basic science. Nonetheless, they (e.g., Comtean positivists) held that the same sort of method that had met with such stellar success in the physical sciences now promised an understanding of humanity (if only it were applied to that subject matter). The kind of intellectual operation at work in this method involved the subsumption of the subject matter under universal laws of nature, themselves to be discovered by empirical study.
Note that those who were charged with social-scientism need not be taken by their critics to have embraced reductionist materialism or, indeed, any metaphysics at all. Instead, they were seen as using their science to plot other people's lives, designing a new social order in which people were treated as pawns to be manipulated rather than as free agents to be let alone. (Such worries about social-scientism are echoed by libertarian animadversions about social engineering.)
Note, also, that since social-scientism was associated with socialism, its critics tended to be conservative traditionalists. However, many who have warned of the dangers of scientism have been further to the left on the political spectrum.
This observation brings us to the second strand in the uses of 'scientism'. The main theme in this second strand is a concern about reductionist programs in which a physical science (such as biology) threatens to gobble up the subject matter of the old humanistic disciplines or of the newer social sciences. Let's call scientism of this second variety reductionist-scientism. This is the kind of scientism that members of the X-Club were thought to embrace. Critiques of reductionist-scientism were more likely to include the accusation that its adherents were committed to a materialistic metaphysics. Clearly, concerns about this brand of scientism were fueled by the advent of evolutionary theory. The concerns were exacerbated by the subsequent rise of social Darwinism. Opponents of reductionist-scientism included not only traditionalists but many left-wing authors as well.
The current use of the term 'scientism' incorporates elements from both social-scientism and reductionist-scientism, although the influence of the latter variety has been more pronounced in the past few decades.