Sort of a weird context to ask for Zulip recommendations, but sure. I find Slack to be a miserably unproductive experience that makes it impossible to keep up with a team (or teams) of any significant size. Slack is good for recreation, not productivity. If the primary purpose of your organization is to be a social group, then by all means use Slack. Meanwhile, Zulip does what I need and does it well. It scales excellently to large organizations discussing broad arrays of topics. It gets out of my way and I don't find myself thinking about it. In that way it's the perfect tool, because it lets me spend energy focusing on engaging with the conversations rather than wrestling with the interface. I guess I'd describe it as natural and unobtrusive, the sort of thing that does its job so well that you only notice that it's there when something goes wrong and you realize what you've been taking for granted.

Note that my experience is almost exclusively with the web client. My small amount of experience with the mobile app is that it's much less polished.