A mature REST API can be a lot of work, and publishing Resources in a flexible way is usually a balancing act.

On the one hand, you want to allow the client to search for information in many flexible ways. On the other hand, you don't want to implement too many operations.

A Search/Query Language for the API makes the most sense - it allows for a single, clean operation while still opening up the API for powerful searching.

REST Query Language Basics

REST Query Language Advanced