As for Slack's IRC gateway, I think it's pretty decent, yes. I personally prefer it to Slack's own client (again, that's just me). But this is what bothers me about Discord, because that doesn't offer an IRC gateway, and I am forced to use their client(s).
And if you're like me - which apparently few are - and you are in many different communities, that use different protocols to communicate, having a bunch of different clients just means more hassle.