There are times where you really don't need an RDBMS, yes. Hard real-time embedded systems are their own bag, I certainly wouldn't want to tell the people who design the systems inside a car how to do their job. As for server logs... they're their own bag, too, although if you decide to roll your own setup there without fully investigating syslog and journald or whatever else, I think you're probably making a mistake here.

But if you're running a website and you have fewer than 100 servers and you think you need to start rethinking the entire relational data model, you may very well be getting ahead of yourself.