One thing's for sure: you can’t sell what your customers can’t find.
Online shoppers place a lot of trust in site search -- they expect it to do the job perfectly. If what they’re looking for doesn’t appear in the search results, they assume the product does not exist on the site.
If results are too broad, they assume your site does not understand them (or it’s broken). When the “good stuff” is buried, they often abandon the site entirely, especially when using small-screened devices.