I've been hacking non-stop for over two weeks. During this time my tech stack has grown to many libraries and multiple services. I've done a lot of experimentation; there are parts of the toolchain that are excellent, and there are parts that are difficult to use, buggy or immature. Here is a review of various libraries and my recommendations. If you're starting with mobile development this might save you a considerable amount of time.
Check out Supabase as an alternative to Firebase, it's powered by PostgreSQL and fully open-source.
The Firebase storage authorization mess is why I personally chose to not use it. Little to no real documentation on it, except "write some rules, then fuzz it!". No, that's not how I roll. Not with data authorization.
But I agree, great ideas and decent execution in general.
But it may not be that no one uses it. I think this comes from a certain JS sub-culture that just doesn't value good comprehensive documentation. It comes across as try it and throw something up, rather then the "Go" programming lang mindset of "study well young grasshopper" then go and program.
Firebase is an interesting choice. It's great for an MVP, but seems to get insanely expensive when things scale up.