I can see that when a task is created, and an assignee is selected before the due date is assigned, it will not allow you to assign the due date when they are unavailable. However, if you assign the due date first, then select the assignee, it automatically changes the due date to the next day that they are available. There is no prompt, notification, alert of this change - it simply changes the due date. This can pose a huge issue if the due date originally assigned is the hard, non negotiable deadline.
It would be great if there could be an alert (as simply as red text or something) indicated that this due date has changed) for those who are not very detail oriented, or simply a project manager who is assigning tasks within a project based on the scope of the job.