The strike is actually aimed at the head for training purposes. But if misses should stop at around your waist level. At that position your torso is straight and balance and your feet are ready to move anywhere. Weight distribution 50:50 and not too much at the front.
Often you will see Uke's striking with their hands starting behind their head, or their shoulders lagging behind the feet, or that the strike stops suddenly upon contact, or if it misses Uke suddenly finds himself with all his weight at the front of the feet or he's leaning forward.
Those ways are incorrect for whatever reason and from whatever art you learn.
So how come even with the 'correct' form, most people disbelieve the power of Shomenuchi? The argument being, no one on the street attacks that way, or that no one strikes the head, and etc etc.
I guess most aikidoka's have 'defeated' shomenuchi so many times, they no longer perceive it as a realistic attack. Regardless, I too will feel the same way when so many Uke's attack with so little understanding.
I wouldn't want to delve into too much of the story that Gozo Shioda used Shomenuchi to break an attackers leg. Instead we shall use common sense. The shomen is really a strike that cuts down, and it could also cut up. Everyone cuts down on a sword, but sometimes you will see cuts going up. Be it a deflection cut or a thrust upwards, the strike essence remains. Also, a shomen using principles of the sword is a very fast attack, one that is essentially used to cover your front as well. Done properly, it will negate frontal attacks. Lastly, even if you train to hit the head... you don't have to aim there all the time.