" If your database requires a lot of relationships, it’s worth modeling out how your database will be constructed in Firebase before choosing to use it. "

Wait what? Modeling your entire application is the first step before you jump on any train. I get it that you say that firebase is different(is it?), but i get the feeling that you are used to using an ORM, right?

I think it's a good rule on thumb to remember that google create products for masses, so the product has to be as broad as it can. they don't create this "simple very easy to use plug and play out of the box" kind of software, but they give you the infrastructure to build your dreams on it. firebase is not some AWS toy, it's a real thing with real problems. you need to tame it and in order to do this you have to know your application inside out.