Search Engine Land: Finally going mobile-friendly, but on a collision course with Google’s mobile-first index [Case Study]
Here's a case study on some of the issues that can arise in a mobile-first world, which we're now entering.
tl;dr - make sure your mobile visitors are seeing the full monty, because that's what Google is going to index.
Google has clearly explained that site owners should make sure their mobile URLs contain the same content as the desktop pages. Those mobile URLs will be the ones used for ranking purposes and to understand structured data.