We still use Slack for internal every-day communications. Personally, I treat Async as a summary of things that have gone on around the company this week so I can keep up to date with different departments (Slack is not ideal for doing this.)
Are most of your people in the Western hemisphere? I find that with Slack I need to pay close attention to the stream which can be a major distraction. Also it can be kind of a pain going through scrollback if I was asleep for a few hours while others are working.
I never feel like I'm overwhelmed keeping track of things, since everything is so accessible thanks to Async. It frees me up to let Slack kind of drift past when I'm not working.
2. We're globally distributed. I'm in the UK, for instance, but we have folks in the Eastern Hemisphere too. I will admit that most folks are in the US so right now, we do skew towards the Western Hemisphere, but that might not always be the case.
Also, in support, we have a summary channel in Slack where we post the most important things that someone coming online would need to know about. For example, outages, recent issues, policy changes, important messages, etc. all get posted to a specific channel in Slack so you can read them _without_ the noise of chat involved.