The big advantage of slack (to me) is that people use channels, and it avoids the "I mailed X and Z, but forgot to CC y in on this", where people routinely selectively email people when really more people should be aware.
You want to document these discussions. In my experience, Confluence, Google Docs, and email are much easier to find things. Specifically because they have a UI which has a unit per discussion (an email thread, confluence doc, google doc) while Slack is a stream of multiple discussions happening at the same time. As such is it very hard to search and also to separate discussions. Discussions can spread across threads while they tend to stay in the same email thread or Google document.
To give an example, let's say I want to have a discussion on a technical design I created. I would probably either create a Google doc and ask for comments or send out an email. If I ever wanted to refer back to the discussion I just needed to find the unit of discussion. It's relatively easy to find an email or google doc.
In slack there really isn't an equivalent. it's possible you could create a dedicated channel per discussion (although no one does) but even trying to find that channel when you have thousands of channels is much harder. Part of the problem is that search isn't very good in slack, but the larger problem is they don't have any equivalent of a "unit" of discussion.
Oh yea and there's also the whole problem that slack only works well internally but there are lots of thoughtful discussions you need with your team and outside organizations.
Nothing but the UI… limited formatting, poor suitability of threads to the purpose, and so on. It’s designed around chat messages.
I wholeheartedly disagree. My inbox is full of emails with subjects and no body, one-line half baked questions, non-responses, incorrect information in a body of correct information that gets lost in the sea of noise, broken formatting that render massive email chains totally unreadable. Often times those massive chains of emails have totally diverged from the initial subject and people on the mail.