The reason people cook brisket here in Texas for 18-24 hours is because it's easier to manage a small amount of heat and keep it there than to turn it up and manage the brisket. At least you can see what's going on inside the fire.
Allow me to illustrate. You can cook a perfect soft-boiled egg in boiling water then standing hot water, and if you are talented/precise/lucky you will get a perfect soft-boiled egg. You can also monitor the water temp and keep it well below boiling for a LONG time, and the timeframe for doneness becomes much more forgiving.
My brother cooked a well-chosen brisket that wasn't even seasoned over coals for 22 hours, and it was hands down the best brisket I've ever had, to this day.