We're coming from Freenode as our only real-time communication so the difference is night and day. Slack is a no-go for many due to not being FLOSS and I'm concerned about vendor lock-in if they were to stop being so generous or run out money and shut down.

Slack's threading model is much worse than Zephyr/Zulip's IMO. Especially on mobile. The streams/topics flow in the "all conversations" view is an incredibly intuitive way to keep track of everything that is going on.