I noticed that only one source was searched, Google Scholar (section 2.1 of the preprint). For a systematic review, Google Scholar is problematic, especially if it is the only source searched. See https://www.ncbi.nlm.nih.gov/pubmed/27076802.

I wonder if you asked for help from a librarian with training in systematic review searching or if sources like Cochrane handbooks were consulted for help in database selection and search strategy development.

See for example: Chapter 7 http://methods.cochrane.org/sdt/handbook-dta-reviews I think might be relevant for your work: http://methods.cochrane.org/sites/methods.cochrane.org.sdt/files/public/uploads/Chapter07-Searching-%28September-2008%29.pdf