I agree with the informal and non-archival nature of Slack. Whenever a serious/word-related decision has been made or somebody drafted a summary for a slack discussion, it should be posted to Trello or some other platform which makes it easier for latecomers to glance through. 1500+ unread msgs after vacation is not fun. Do I miss anything important and work-related? Or just some random cat pictures and movie critics?

In addition, it's hard to track progress with Slack. It's multi-channel, multi-topic, multi-project, and etc. I mean, Slack is great for quick discussion, but terrible at keeping records.