How is Google Scholar compared to other academic search engines and catalogues such as ScienceDirect, Scopus, and universities' own online catalogues--in terms of the following?
- comprehensiveness of search (i.e. can fetch all the ever published relevant titles, whether the user have access to full-texts or not)
- accuracy and precision of results
- advanced search features
Are there good reasons for (or against) using Google Scholar as the only literature search tool for PhD research?
Does the answer of (2) depend on the research discipline?
There has already been much scholarship on the comparison of different academic search databases. I leave it to you to ascertain the usefulness of Google Scholar versus all the other usual suspects. Here, here, here, here and here are some of the prime literature on this topic.
The real answer of course, to all your sub points is, it depends.