Async Apex consists of all batch Apex, future methods, Queueable Apex, and scheduled Apex. There is a rolling 24-hour call limit of 250,000 or the number of user licenses in your org multiplied by 200, whichever is greater.
Whenever you hit this boundary, it could put your business processes to a halt as the functionality just stops working until the 24-hour period is over.
Please implement a warning mechanism equal to that of API Usage Notifications. So a treshold percentage to be specified by the end user, and a user to send the notification to.