Fries shouldn't be a difficult problem to solve. Donut Machines have been doing something similar for a long time (production line deep frying) and I imagine that Fries wouldn't be very similar if you wanted a consistent output.
The biggest problem was having demand that continues unabated; a machine like you're describing is used in industrial food production I believe.
To automate everything and have it hot and ready is a really hard challenge. Fries take about four minutes to cook. A burger between one and three. While you can compute an average car order, any given set of cars will order significantly different amounts of food. Get it wrong and you are forcing people to wait (what we are trying to get away from) or wasting food (bumping up costs). This optimization was the thing McDonalds spent the most time optimizing when I worked there years and years and years ago.
On the other hand, I'd be happy with waiting if I could input my order and I knew my order would be correct.