This is why they say the most "knowledgeable" aikidoka is a 5th kyu
Hell when I was there I wanted to show everyone how cool aikido was. And every time I just frustrated myself.
If you havent noticed, the philosophy of aikido practice assumes that both persons know what they are doing to fulfill their role of tori and uke. Otherwise all the breakfall and ukemi make no sense at all, because no untrained person would do that "I m gonna take this extremely scary highfall because I dont want to lose my limb"
So for a trained partner shihonage would work because he knows that if he resists some realll bad things can happen. Either you switch to ude kimenage which is a brutal arm lock/break, or you extend your arms, crank his elbow up using your core movement and turn around with an immediate cut (this is how we "force" a shihonage in my dojo) The only way to get out of this with all limbs intact is a breakfall. Besides there s always place for a fatal atemi, i.e you can break his kneecap from behind with your inside leg when you start turning his body. This is why we rarely see a real atemi in aikido I think. They are supplementary but they are meant to be disabling given the precise positioning of aikido techniques.
All the above are good reasons to not do aikido on an untrained person. Because they will resist and you will be forced to go brutal on them if you want to protect yourself.