Matrix the protocol is pretty awful, and I'm not a huge fan of the "official" clients, but YMMV. The real problem with it though is that unlike IRC and XMPP the protocol itself isn't developed in a sustainable way by a standards foundation with a reasonable funding model. The protocol itself doesn't really matter that much though as long as we pick one that will be maintained and won't just go bankrupt (so IRC or XMPP depending on what you want to build) and then build nice services that support it. Remember, Slack is a service and a protocol, XMPP/IRC are just protocols. You still need good services like conversations.im or Freenode built on top of them for it to be usable. Unfortunately, we don't have many good services like Slack that have commercial backing but use an open protocol.