Yeah, nothing about IRC is intrinsically difficult. There's some things that are much easier on Slack, like archive search, but how many people actually use that?
If you tell people something is hard to use, they'll believe you. If you tell them how to use something, they'll use it...
The really big thing for me is persistence. If you want your client to stay online when you are not, things get very painful. Bouncers are a pain to setup, command-line IRC clients are awful, and I've seen countless people horribly confused by screen and tmux.
With slack, persistence comes for free. The only thing I've seen that comes close is irccloud, which is in my opinion an excellent competitor to slack. In the end my organisation went with slack because it's nice to have everything grouped and billed to the organisation, as well as being even simpler for end users to join.
If you tell people something is hard to use, they'll believe you. If you tell them how to use something, they'll use it...