The water is forced against the sides of windows, doorways, corner boards and any vertical element that stands in its path. Without very careful detailing it will leak into the building at these points. I'm not crazy about horizontal board siding in windy or exposed locations either. If you have knots, splits or defects in the tongues or grooves you run the risk of allowing water to pass through the siding when driven by wind or gravity. Good house-wrapping details are critical.
Vertical installations provide better protection.
Panel Siding Another common failure involves improper installation of plywood or panel siding. Problems typically occur at the horizontal joints between upper and lower panels. Often plywood panel siding like T-111 is used to economize so structural sheathing is not installed beneath the siding panels. Very often house wrap is not installed either. These connections are not weather tight.