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.