My research group (at MIT) also uses Zulip for messaging. While I agree with everything jremmons said (hi john), it's important to note that their mobile apps

My research group (at MIT) also uses Zulip for messaging. While I agree with everything jremmons said (hi john), it's important to note that their mobile apps are so bad that they're basically unusable - there's a particularly aggravating bug in the iOS app where if you don't open the app for a while, it forces you to load and scroll through days of messages to read the most recent ones.