Since nobody has actually answered this, and I seem to be the only somewhat-active person here who's used PS!, I'm just going to throw this out there:
In most things, PS! has basically the same functionality as IRC, with two exceptions:
Logs. While you can still copy/paste logs on Showdown, afaik there is no convenient way to store logs without actively copypasting them, unlike most IRC clients which have an option to automatically record all logs.
Private(i.e. mafia factions) rooms. While Showdown has group chats, these disappear after (I think 6 hours?) if nobody talks in them, and require either a room mod or global voice to create them. (although, at any time when more than two mafia needed to discuss, they could easily start a battle and use that for that specific time) Really this shouldn't be a problem, the system is just a lot less flexible but there shouldn't be anything stopping faction discussion.
Other than those two(room flexibility and logs) PS has pretty much all the functionality of irc unless I'm somehow forgetting something big.