I never said underhand was impossible, just that overhand makes more sense and correlates with historical sources (from early medieval all the way back to classical)

Some criticisms I have of many reenacting techniques are that we don't have 100% accurate equipment, due to safety mostly, and although the aesthetic effects are minimal, the way this affects the fighting technique is noticeable. Another is that, even the most devoted of us do not get to go through both the fighting drills and the day to day labours that would have a huge impact on our physiology. A final, and by far the most poignant, is that you're not facing death. This is something that affects all sorts of simulation, from reenacting to video games. Knowing that death is a very real and, depending on the warrior's religious views, very final problem.
I mention this because in both RTW and in reenacting everyone acts far too eager and gung ho. Reading tales of heroics, we often forget that these stories are there to help people overcome the fear they would face. In truth, we have no idea how the people of this era fought.