The biggest problem with Slack, by far, is that it is effectively ephemeral. An enormous amount of valuable content in Slack quickly becomes undiscoverable for all practical purposes. This creates significant challenges if history matters and this only gets worse as the number of people on it grows. In every organization I've participated in where Slack is the central "everything box", they had to invent parallel processes and systems so that things don't get lost in Slack.

Slack should be treated like the super-IRC that it is, it is poorly designed to be the nominal system of record that people try to use it as.