When do you not need a real time database for a feature that feels real time?

I've been thinking about features that users see as real-time, like a live comment feed or an activity stream, but they might not actually need a super fancy real-time database. You know, sometimes just polling really well with a short interval, or even using server-sent events for less important updates, could give people pretty much the same experience. And you avoid all that extra headache and cost of something like Firebase or RethinkDB. I'm really curious to hear what other folks think. How do you decide when to use a real-time database versus a more old-school setup with a bit of a real-time feel? Are there certain kinds of data or ways people interact that always make you lean towards a dedicated real-time solution?