Surprised no one here has mentioned Twist. https://twistapp.com/ We're currently evaluating it in lieu of Slack on our team of ~10 people because of the vast array of issues Slack has. I don't know if it's something that could be considered for larger organizations, but the way it puts together channels and threads is very intuitive, and cuts out a lot of noise for individuals.
In terms of Slack's misgivings, I don't think I've used a more incredibly bloated, buggy, slow electron app. Multiple team logins with no centralized account is _insane_ to mange across teams, this single point frustrates me and most people I talk to a lot. Not to mention the more teams you have, the more your electron app slows to an absolute crawl. The notifications leave a lot to be desired in terms of granularity per channel. They've done a really terrible job on "threads", the UI around it is messy and confusing, and the concept doesn't inline well. Those are just the most apparent daily gripes.
In terms of Slack's misgivings, I don't think I've used a more incredibly bloated, buggy, slow electron app. Multiple team logins with no centralized account is _insane_ to mange across teams, this single point frustrates me and most people I talk to a lot. Not to mention the more teams you have, the more your electron app slows to an absolute crawl. The notifications leave a lot to be desired in terms of granularity per channel. They've done a really terrible job on "threads", the UI around it is messy and confusing, and the concept doesn't inline well. Those are just the most apparent daily gripes.