Hangouts API is discontinuing, and the app is buggy as heck. We've already moved away from it and are experimenting with in-house solutions or looking for alternatives. Slack is too pricey, everything else seems shite.
You mean Google is discontinuing the API? (They had an API? They removed XMPP federation and eventually support, if that's what you mean).
The libpurple/Hangups projects for chat clients is the work of reverse engineering the existing Hangouts protocol (which apparently heavily relies on protocol buffers).
