Can you give me a concrete example of were you'd contort if you were to use Redis versus Postgres? BTW, I'm just trying to really find an actual proof as opposed to mere anecdotal knowledge.
The model subtly breaks even on simple questions like "I want to know all users who are currently logged in and the contents of their feed right now, with the last 5 messages they sent". You basically have to do multiple requests, MULTI, or write a lua script to get all this to happen in a transaction (so the state of the world is consistent). Such a request is trivial in relational databases, though performance of the query may not be.