Unity Multiplayer, Phaser+Firebase

Create real-time, networked games

Unity Multiplayer is the simplest way to create multiplayer games for Unity. It’s fast to implement and highly customizable. Unity-provided servers ensure that your players can find and play with each other.

FREE: Unity Personal/20/Concurrent players

https://unity3d.com/unity/features/multiplayer

VS:
Phaser, … + Firebase
100 Simultaneous connections

Unity and firebase : https://firebase.google.com/docs/unity/setup