I do not see how that comparison is unfair. Terminal IRC clients, for example, accomplish the same exact functionality as Slack in a curses UI in a vastly faster way.
I've used slack on mobile and I find it worse than the desktop version.
It starts slightly faster (10 seconds vs. almost 20) but navigating through channels is bad (and slower) and threads are completely unusable. It also runs my device hot like nothing else.
The faster startup speed isn't a big victory for me because it's still a factor of 10 away from what I would consider acceptable.
The "best" interface for slack, IMHO, is the web version, provided that you are already using Chrome.
* DCC is still unreliable.
* There is no audio connection option, which is quite popular in 2018.
* Channel history management is ad hoc
* Authentication is done in band, in plain text.
* "secured" channels rely on this bad authentication, and if they don't (perhaps electing to manage it themselves) network flaws can completely steal your channel.
* IRC isn't even particularly open source, many servers and networks have private patches that are not shared publicly.
IRC isn't better than slack. I agree with you that it's confusing why people suggest that it ought to be.
Also, most IRC networks are controlled by entities even more inscrutable than Slack's executive team and board. I can go look up who runs slack, I cannot actually find good details on who runs any given IRC network. I have no idea what I'm dealing with or how they're using my data. I have no legal recourse if I do discover bad behavior, and I'm forced by the ossified protocol to keep using their insecure authentication mechanisms which make abuse trivial.
Ah, the good old days of the Internet. You make me so nostalgic.