Generic search engines such as Google Scholar could be useful to identify research from publishers not well covered in the other databases targeted for the study. However, using Google Scholar has definite limitations that needs to be accounted for:
It is difficult to undertake large boolean searches and to manage the results. It is necessary to be very selective in the use of Google Scholar and use it for it strengths to identify gaps perhaps.
The scope of the search engine tend to be less clear than the case of specialist databases in inclusion of sources as well as in ranking and displaying (as well as repeating) the searches and results. Search engines such as Google Scholar is further not manually indexed such as specialist databases.
It may be necessary to include more than one (partial) searches to reach the objective as set of the project manager and project team.
A balance between sensitivity and precision could be harder to be achieved and careful testing first would be advised. The search strategy should be carefully documented so to make the decision making trail more visible.