We've been using Mattermost (I have not used Zulip but it looks superb) for about a year. The UI/UX of the desktop and mobile apps is a bit kludgy but functiona

We've been using Mattermost (I have not used Zulip but it looks superb) for about a year. The UI/UX of the desktop and mobile apps is a bit kludgy but functionally it was a breeze to setup, runs on a very low-cost instance, and has had 100% uptime. And most importantly, we're in control of our data unlike cloud-only offerings.