The problem I see with message apps is that it's like email; you really wished you could host it yourself and fine tune things (as well as make sure nobody is eavesdropping). But you can't have it down or (worse) performing at half capacity. It needs to be up all the time with almost perfect quality.
Sure you can set something up yourself, but you'll probably struggle with maintaining a decent QoS, and if your team is any good, they probably won't allow that to go on very long.