I have a smartlist that contains my tasks for the day. The list contains overdue tasks, tasks due today, tasks with today's start date, and untimed tasks with a specific text tag. Works fine. The app icon symbol is also derived from this list. On the Mac and iPhone, all tasks are counted except those that are in the list because of the start date. The number on the list itself is correct. The Apple Watch works differently (and from my understanding correct); the AW complication displays ALL tasks of the list in the complication, even if the tasks with the start date.
I haven't found a setting variant that displays the same number of open tasks on all three devices, feels a bit confusing...