All angled shots, for archery or firearms, will produce high impacts if you don't account for the angle. The greater the angle the greater your error.
Gravity is the force that pulls the arrow or bullet down. When your projectile is moving horizontally, as opposed to moving at an up or down angle, then gravity pulls the projectile straight down for the entire horizontal distance. When the projectile is moving at a inclined or declined angle then you have to calculate the related horizontal distance rather than the straight line of sight distance. The line of sight distance (the distance of a line drawn from the shooter's eye to the target) is the same as the hypotenuse of a triangle (always the longest leg of a triangle) but the actual projectile drop is calculated by measuring the length of the horizontal leg of the triangle.
I only shoot firearms and for a rifle you don't have to worry about angled shots too much unless you are aiming up or down by 25 degrees or more and the range exceeds 400 yards or so. Time of flight has an effect on how much error you will experience, faster projectiles will have less error so I'd assume that arrows, being slower than bullets, will suffer more from this issue, especially at longer ranges.