Then the other pain point is the "joins" use case; right now we do the equivalent of fetching all comments & users, then doing an in-memory join. Ideally, we could craft a single request that just says "get the 10 latest comments on this market, plus the associated avatars" without data duplication and without doing a bunch of up-front thinking about exactly how to structure indexes.
My hesitation with relational DBs comes from the mismatch between client data model (loosely, JSON objects of pointers) and how it's represented in the DB (in a row); plus the requirements of needing to specify your e.g. indices up front, and annoyance of doing migrations. I'm hopeful someone's found a graph-type solution to work really well for them!