I use servers extensively, for several communities (some small like my weekly RPG game servers and my book club, some much bigger like the AI art servers I'm on), all of which have multiple channels which are used for different things. Correct me if I'm wrong but afaik Signal (and Molly which I'd previously not heard of) doesn't do that
It being closed source is unfortunate, not being very privacy friendly is certainly not good but I don't think there have been any major leaks have there?