UI is not great.
Turns out, it's a bit of both, and trying to use it as any single one of them, will probably make it harder to use.
Basically, I treat it as a live chat where it's easier to participate in different topics at the same time, Slack on steroids where everything is a "Thread" basically.
Alas, Flowdock is dead now and I've learned to embrace Slack as the better remaining alternative over Teams or Google Chat.