i haven't used irc for about 20 years, but i don't remember having as many problems as i do with slack
#quakenet
#quakenet
On IRC you had netsplits where half of the people could still talk to each other and the rest were disconnected. I would guess it was equally rare (back when I used irc more, 8 years ago?) as outages of "centralized" services today.
On slack the only indication that anything was wrong for the past 2 hours was that my co-workers weren't responding.
So it's possible that slack has as many faults as IRC or more but we don't know because the interface is so dishonest/corrupt.