For example, what do you find hard to find on Google search engine, or some other custom search engines (e.g. on e-commerce websites)?
Not to mention that although popular search engines should largely the same, they are slightly different in relevancy. (And somehow Google Search is slightly better...)
Perhaps this topic is also about full-text indexing and NLP in general.
There are also other aspects in relevancy, like indexing the images' meaning? videos? colors itself?