If one correctly leverages hypermedia, you may and most likely will have heterogenous responses and relations. Side-loading relevant relations is highly encouraged and is a core component of efficient API design.

To your credit, I believe most of the information out there about REST and REST-like APIs discount how much work actually goes into building a proper REST API with hypermedia -- so we will always have half-baked design patterns that solve problems when doing REST incorrectly.