I just tried it as Slack is such a bug-ridden piece of crap these days, unfortunately it just loads the Slack website in a view, so you get all the fun of Slack's bugs in a slightly crappier interface.

Edit: I should say it's not Franz that has the crappier interface, it's the Slack webview, Franz's interface seems pretty nice. However, if something wants to be the biggest app binary on my Mac outside of Xcode and Logic Pro, it needs to offer a lot more than this.