I used it daily for decades, but let's not forget netsplits, nick collisions, zero log history (without a bouncer, and then you need a place to run a bouncer!), no file-sharing in a NAT world (dcc stopped working a loooong time ago), and no integration with any other apps, and on and on.
Don't get me wrong, I wish there was an open-source project that built a big group chat system that worked as well as slack and discord do, but it's a joke to say that irc is better than slack.