The two biggest pain point that I see is: 1. Because there is no real account management you don't have any proper authentication, which make administrating a channel real dodgy (even with network provided bots).
2. No offline history: You have to have a client/bouncer running 24/24 if you want history.
One thing though, because of how IRC work, you don't have the problem that other protocol like XMPP faced with multy-device sync (there is a extension for that in XMPP but, like almost every extension in XMPP, not many client support it).
Also, nowadays we should consider end-to-end encryption standard.
I feel like IRC is in the same space as email: It's a very good technology that just lack a few feature to be perfect but any project that try to replace them just end up over-bloated with features ...