Vertical punchers say that the elbow bows outward in horizontal punching. Vertical punchers also say that in a horizontal punch the elbow is turned outward leaving the body more vulnerable. Make a fist and hold your arm straight out in front of you in a punching position. Rotate the fist to vertical position, then back to a horizontal position, etc. and observe the motion of your arm. The fist and wrist rotate 90-degrees, while the elbow rotates very little. No matter the reasoning used by vertical punchers to explain how the horizontal punch leaves the body more vulnerable because of the rotation, any increase in vulnerability is negligible since any elbow movement is negligible.
When one stands in a standard fighting stance and extends the lead arm forward in a horizontal punch, there is a slight outward bend in the elbow because of the way the arm is constructed. When the fist is rotated to a vertical position, the natural bend is still there, except it is now pointed downward. There is no more body mass behind a vertical punch than there is behind a horizontal punch, since both have the same alignment with their common point, the shoulder. When punching with a horizontal punch, one could choose to bend the elbow more, but one could also choose to do the same thing when using a vertical punch; excessive elbow bend is not desired in either punch. The elbow’s natural bend is not affected by whether the fist is held in a horizontal or vertical position.
Vertical punchers say that, with the elbow angled outward, it is more susceptible to injury than it would be if it was angled downward. They say the outward elbow also adducts the shoulder leaving it vulnerable to anterior dislocation. This may be true, but it only matters when you are punching in slow motion, as is done during a demonstration. At full-speed, it would be unlikely that the elbow would ever be hit in such a way to cause damage.
Vertical punchers say that angling the elbow outward exposes the floating ribs and the pressure point in the pit of the arm. When you arm is extended in a punch, the difference in coverage offered by a downward elbow and an outward elbow is negligible.