You get a gravity boost from running downhill, so can go faster. But, landing force is greater. That is, running downhill has higher impact than running on level ground. This means more stress on the bones and joints.
If the hill is steep, running downhill is an eccentric exercise for some of our muscles, such as the quads. In eccentric exercise, a muscle lengthens, but lengthens while resisting lengthening tension.
To help make that clear, let's say you are doing bicep curls. Keeping your upper arms in position, you raise and lower weights using your forearms. Your biceps contract to raise the weights. But, when you lower the weights, you don't just let them fall. You lower them slowly. When you lower the weights, your biceps are lengthening, but at the same time, they are preventing the weights from falling quickly. Lowering the weights is a type of eccentric exercise.
Try having some runners go up a long, steep hill, and other runners go down the same hill. If they are not trained for long, steep hills, both will hurt. But, if the down hill runners ran hard, they will hurt more.