If you're going to use a tool to help you with an important part of your research, it helps if you know a bit about how it works.
Searching for literature is a major, time consuming, and vital part of any PhD, so your choice of search tool matters.There are some major drawbacks to using Google Scholar. That doesn't mean you shouldn't use it, but you should know what its weaknesses are.
The Google Search Algorithm
When you do any kind of search through google, their search algorithm decides in what order to show you the results.
It's an incredibly sophisticated system, taking into account all kinds of measures of importance and relevance for each search result. But here's the problem... Nobody knows what the algorithm is. So you don't know how they've sorted them, and you have no control beyond selecting the search terms and what years to search.
If I search for "scanning tunneling microscopy", then I recognise some of the top results (including the nobel-prize-winning inventors of the technique). There are papers there which have been cited hundreds or even thousands of times. So far so good.
But there are articles there that (with all due respect to the authors) have no business being in the top 10.
Why Google ranks a paper with 3300 citations at number at number 6, and a paper with 21 citations (from 1990, so it's had plenty of time to have more of an impact) at number 7, is anyone's guess.
This stuff matters. There are over 200,000 search results, so if google is filtering and sorting the results for me, I'd first I'd like to know how, and second I'd like to be able to play with the settings to sort results the way I want.
The advantages of Google Scholar
Well it's free, so anyone anywhere can use it (even if you have to pay for access to some of the results).
And of course you can change the search terms you enter to get more specific results (but that's not really an advantage as you can do that with any search engine).
I always used Web of Knowledge, which gives far greater control and transparency over search results. It requires a subscription, but if your institution is registered then definitely use it.
The key thing is being able to control how search results are presented to you. Leaving it up to Google is not PhD-level thinking.
I'm going to throw this one over to you in the comments section. What tools do you use to search? And why?