Graph DBs are great for intersection queries (AND queries). In fact, DGraph is designed to do those really fast; and supporting that via GraphQL is in our roadmap.

https://github.com/dgraph-io/dgraph/issues/1

In general Graph DBs are great when you have many "kinds" of things, which would require many many tables in traditional databases, and lots of interlinking. Those scenarios are ideal for Graphs, because many different kinds of things can be interrelated to each other easily, and be queried seamlessly. In other words, the schema for graphs is very fluid.