Yes, Zulip has the disadvantage that the UX isn't exactly the same as every other chat program, so it takes some getting used to. The UX is great, it's just different enough that people don't grok it immediately, and that's too much for people unless you start with a core that's using it and grow the company around that (hard to do with an already established company).

I'm afraid I don't know of any videos, but I tell people "Zulip is exactly like Slack, but everything is a thread, and you can zoom in and out of threads" and find that that's basically enough for them to get it. Maybe try that with a few people and see?