Google Scholar indexes almost everything that Web and Science (as well as PubMed, Scopus, etc.) also indexes. So, Web of Science is merely a subset of Google Scholar. Web of Science offers almost nothing meaningful if your aim is to retrieve all relevant literature.
It is true that Google Scholar indexes quite some junk. But who decides what is junk science and what is not? Should this be the job of Google Scholar? The claim that Google Scholar is a “potentially dangerous database for anyone doing serious research” seems invalid to me. Why should Google provide the filter for the serious scientists? Can’t the serious scientists judge for themselves which paper is valid, and which is not? Isn’t this actually the job/obligation of scientists? If scientists want Google Scholar to filter out the junk for them, it means they apparently cannot decide for themselves which papers to include in their literature review.
If your goal is to do a literature review or meta-analysis, then the grey literature (e.g., government reports, conference proceedings, PhD/MSc student theses) need to be retrieved as well, otherwise you are contributing to publication bias. Web of Science misses all of this. People who only rely on Web of Science (PubMed etc) wrongly assume it is sufficient to rely on a subset of the overall scientific enterprise
It should be noted that Google Scholar indexes papers that appear online within a day or three; other indexing services take months or more.
