literature - Google Scholar vs other academic search engines

1. 
   

   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
   
   
   • usability
   
   
   
   



2. 
   

   Are there good reasons for (or against) using Google Scholar as the only literature search tool for PhD research?

   
   


3. 
   

   Does the answer of (2) depend on the research discipline?