CometChat works seamlessly on web, mobile & desktop! Your users can be on any platform and communicate with each other.

Check out the demo here: https://www.cometchat.com/demo

Use https://matrix.org if you don't want to depend on the infrastructure deccisions of a third party.

PubNub. We have fantastic uptime and do over a trillion transactions a month flawlessly. We also do some cool blogging too. (my job :)

http://www.pubnub.com/

Firebase is a great platform for building chat functionality

Twilio's chat SDKs https://www.twilio.com/ip-messaging

You can try cometchat.com which is a self hosted. So no such issue.