The problem is response delay. If you need 10 rounds of back and forth to communicate something then your total time is 10*delay. Moreover, at some point people start to forget the context so the total time goes up even more and quality of responses goes down. If it takes 6 hours for a response then you can spend days waiting for a relatively simple point to be hashed out. Especially if there's multiple people whose input you need who are all working at different schedules.

Clearly, having 0 work hours in common with the rest if the team is a problem. But what if you have at least 2-3 hours per day? You can already discuss a lot of things during that time.

I get that asynchronous does not always work, but it has some benefits, especially for tasks such as architecturing an app. I'd be worried if such discussions would happen only in an oral fashion, or in chat channels.

I find this often boils down to people being unwilling to read/write very much. If people insist on reading and writing no more than a single sentence at a time, async communication will be very difficult. If they’re willing to provide full answers to the question, it’s not so bad.

Good documentation is also invaluable, and good documentation being 1:N is often more valuable than a 1:1 synchronous discussion for certain things.

That's assuming people bring up the patience to wait for a response - a rockstar developer type is inclined to go "screw that, I can whip this out in a couple hours", so by the time others have a chance to think about it it's already done. Not too late to challenge it, but it'll feel like wasted effort then.

Sometimes people need to feel the sting of a wasted effort to understand that they work on a team. Don't want to waste your time? Don't waste the team's time.