I have looked at the rust Zulip forums, which are bulky. But with moderation and rules and having people on the autistic spectrum [citation needed], it perhaps is usable for large organizations. Just kidding.
We are using Zulip for 300+ members in a makerspace, and at 40 members, we were not happy. Scaling to 300 never broke not being happy, since we all hate the UI ever since.
I cannot re-open Zulip threads, which are also issues with an atomic "solved/unresolved" state, unless I have elevated access. It is not a true forum like PHP forums, where we ask people to name threads, and you might just skip reading more than the title, or locate interesting threads by activity and find stickies about important announcements in a pull, not push, way of doing things.
It instead is a chat where a thousand group chats are open, and no once wants to read any of them.
If they wanted to re-invent forums, they should have cloned the "discourse" web app/forum. Still looks like shit on every platform, mobile or desktop, but at least does not break down on mobile.