It's the good ol' "use the right tool for the right job", but with an added "but also use the tool well". Throwing Firebase behind any UI is certainly not a mature decision. I will also say though, pretty much everything that applies to NoSQL, applies to Firebase as well. What I mean by that is you should not look at it from a Firebase perspective but whether NoSQL is the right type of DB for you. I see NoSQL being great for many micro-services, though certainly not all. I can also see it being great for proving out concepts or even early versions of a live product as long as you have a migration plan should you need to migrate.