i think what tsukuyomi does is.. it does not inflict physical damage.. but it's totally strong genjitsu that you can't get out of it by just telling your brain it's not real... he gets them into a world where he can create almost anything.. adn tricks ppl... with kakashi.. i do not really think he actually was damaging kakashi's soul.. prob just was tricking him... but it seems to be a quite strong genjitsu like i mentioned