The simplest advice to engineers would be to not think elastic search as some black box that will solve all of your search problems. In fact, if you've never implemented search before it's the last tool you need. Postgres full text search is all you need. The most important thing in search is to surface relevant results and no one can quantify relevancy. It's as unquantifiable as it gets in technology. You need to understand what results are relevant for your users and find a metric that would work to rank accordingly.