What's your experience with async standups?

Depends what you mean by standups, if you are talking about a status meeting it probably works for most teams.

If you are talking about Scrum though, a stand up should be an opportunity for the team to assess where they are at in the sprint. Then discuss what they need to do to achieve the goal and make a plan for the day that puts them on the right path. Doing this asynchronously is a pain.

We do them. Seems to accomplish the goal. It's better for reflecting and less oppressive to be there at the same time every morning. Make it mandatory, talk about intentions for the day, blockers and PRs that need reviewing.