Harmonic Drives (HD) are commonly used in robotics because of zero-backlash and compactness. This makes them ideal for several mechatronic applications. Despite their wide use, there is no compelling literature on their performance, and to date it is not clear how they fare in a complete actuation setup. In this work we evaluate the joint structure we currently install on the iCub humanoid robot comprising an HD speed reducer showing that their efficiency is not optimal.