We use Firebase at work (and I use it for my personal projects) for several reasons:

1) it is not just real-time DB with subscriptions but a lot more (storage for files, push notifications for mobile, cloud functions etc.) so it allows to build quite a robust solutions, but still possible to use just a minimal set of what you need

2) In most cases it's pretty cheap (unless you messed up you DB structure, reads/writes etc. - could be problem for a lot of traffic - so in such a case ready pricing and related guides properly), for side projects basically free.

3) offers free hosting with SSL certificates for static files

4) you can bootstrap functional prototype really quick and for the production, you do not need to worry about scaling.

I recently used PubNub to implement a realtime leaderboard into my Unity game. It was easy and straight forward to get up and running. I choose Pubnub because they had a Unity SDK and an example project which showed me how to use it. I downloaded the example project and tweaked it to fit my game.

In their documentation I found good information about what I needed to know. Also, their support is very helpful when you have questions.

I used PubNub because we needed a real time server for our product. Even though we selected a fixed amount paid version, the amount they charged was incredibly high, 8x of the fixed amount. We could not understand how did they do the calculation and there is no easy way of understanding it. Regarding to the analytics they have, we spent millions of messages with few hundred active users.

Then for the second month we tried to change our architecture to decrease the messages but nothing changed. Then we wanted to get some support from them. We arranged a meeting and the person who joined the meeting didn't even have permissions to see our usage, so it was waste of time. Then they arranged another meeting WEEKS after which didn't solve anything either. As a result, we stopped using it and implemented our own simple pub-sub server.