WunderGraph offers a unique solution to join data from multiple APIs, allowing developers to combine data from different sources into a single GraphQL API without having to write custom code. By using WunderGraph's server-side only GraphQL approach, developers can easily integrate APIs and expose a secure REST API with a typesafe client. This solution is particularly useful for building microservices and combining data from different databases, such as PostgreSQL and MySQL. While it has some drawbacks, including the need to deploy a separate WunderGraph server and learn new concepts, it provides a flexible and efficient way to join data across APIs. Additionally, WunderGraph's JSON Schema-based approach allows for easy modification of the response using "lodash" directives, which can help flatten the response and add new functionality.