Limits of Scope
For librarian William Badke, one of the main frustrations with Google is its limited content. It often doesn't search the most important sources1. Most experts would agree that a single search engine captures no more than 16% of the entire Internet2.
Consequences of Habit
Another tendency of Internet researchers is to use the same search engine for every search3. Applying the information above, it becomes evident that repeatedly relying on a familar search engine severely constricts your awareness of the information available on a topic. This tendency towards the familar goes "hand-in-hand" with not using the advanced features of a particular search engine.
1See https://login.cscc.ohionet.org/login?url=https://search.ebscohost.com/login.aspx?direct=true&AuthType=cookie,ip,uid&db=a9h&AN=96354060&site=ehost-live
2L. Graham and P. Metaxas, "Of Course it's True; I Saw it on the Internet," Communications of the ACM 46:5, May 2003.
3See https://cacm.acm.org/magazines/2003/5/6838-of-course-its-true-i-saw-it-on-the-internet/fulltext