Slack is fine for transient communication. That is, it's fine unless you might need to dig up a particular conversation a year after, or five years. Actually, finding stuff as old as one week can be problematic already.

Maybe for you this is not important. For me it has proved to be key many times.

Also, filtering. You cannot withstand the same stream of info and notifications from Slack as you can from email, because there are no filtering and classification rules. Channels help, but in a limited way.

Slack works well for casual and ephemeral communication. For other use cases, you need a serious tool like email.