I think scientism is best summed up as the fallacious idea that science is a priesthood rather than a process. Some people really struggle separating the politically neutral idea of using the scientific method to inform policy with the politically charged idea that a scientific technocracy is a desirable state of affairs in my opinion, a problem that's only got worse in recent years.

Science is a powerful contributor to the sum of human knowledge, but it's absolutely not the only pillar of human knowledge and there's a lot of people who think that scientific advancement means we can just do away with philosophy, politics, and other fields like that altogether, as though one day with the right equations all of ethics or culture is just going to pop out of physics. These people don't just throw the baby but the whole nursery out with the bathwater in my opinion!