The extremely poor JOIN functionality is what turned me away from Firebase a couple of years ago.

I come from 25+ years of SQL, and Firebase was my choice for essentially my first 'real world' NoSQL project. I was enjoying it right up to the bit where I had to extract data from 2, and 3 data tables at the same time in a single query. Couldn't believe how verbose and painful it was and it made me drop NoSQL for a while before more recently going back to it (with RethinkDB).