One of the most important reasons for my doubts of aikido's effectiveness lies in the difference between a practice situation and a real situation.
The uke is a calm human being, moving smoothly without any intent of causing injury.
The man with enough anger and ferociousness to want to beat another down, and perhaps to death, has transcended the realm of humanity and become an instinctive animal. A viscious, unfamiliar, frightening, erratic, epinephrine-filled, testasterone-filled, and endorphin-filled animal. This animal will scare the ki out of most anyone.
In the dojo, we certainly learn to overcome some select fears, but we do not learn how to overcome the freezing fear of a ruthless attacking animal. We practice comfortably with fellow loving aikidoka, but I feel there is nothing comfortable about a life-threatening situation. I think I'd end up in the fetal position before trying to smoothly guide the enemy's aggressive energy, then attempting to lovingly change his heart and intentions. I like the ideal for a peaceful resolution, but I lack the faith in its utility.