For many developers, using a cloud-based real-time database may be way out there and not even worth consideration. Of course, it also very much depends on the type of game you are developing. You likely wouldn't want to use a real-time database for a FPS, but fortunately, Unity has excellent options for that. For a location-based game, though, a real-time database makes ideal sense for all the reasons we mentioned earlier.
Note
Even if you are not making a location-based game and one that doesn't require ultra-high performance updates, you should consider Firebase. The free version of Firebase real-time allows 100 concurrent users and the first level subscription provides for unlimited concurrent users. Other multiplayer platforms generally start charging after 20 concurrent users.
We won't add Firebase as a multiplayer platform to the sample game we developed over the course of the book. That would likely need several chapters or even a book on its own to...