We recently published a mobile game that uses Colyseus as a communication layer between client and server. Unity was the framework that we used in client side.

Its google play link: https://play.google.com/store/apps/details?id=com.mkcelikbas.OnlineWordChain

Our general impression towards the Colyseus is highly positive, although we needed some hack in some parts.

Here is the most prominent problem we faced ( not so important though)
client.close() does not trigger anything in client side and for the server side when room.close() is executed, thus we needed to send a message first that says the connection will be closed.

I'd love to hear more about your experience in the client-side, since the Unity client is not so easy to use in comparison to JavaScript, due to the insane amount of typecasting necessary on C#.

client.close() does not trigger anything in client

It's likely that this one can be fixed. Unfortunately, the Unity client lacks some good care.