This kind of marketing turns me off before I even get to hear most of their message. Especially when they say they are better than slack because they are "free". Free just means we have a huge burn rate until we can figure out how to make money. It doesn't instill confidence, it makes me weary.
I'm giving a go at their application and I'm right really pleased to see how easy a potential complete transition from Slack would be done. You provide your Slack's API Token and everything gets imported from the look of it: channels, history, snippets / files, etc...
while it looks promising, I don't like the idea of the "stream" for everything. With enough content old stuff gets lost, this goes for chat or news feeds. Traditional forums works because on a single page, i can see a list of sometimes a 100 different posts and skim through them and see exactly what I'm looking for. Search work if you know exactly what you're looking for and of course if the search works well.