I like to see shomenuchi as uke's defense to a pre-emptive strike to the head from tori.
If tori's timing is perfect, he will knock uke in the head and it is over.
If tori is a bit slower, uke will be able to defend his head by raising his arm, when uke's balance is broken, go through with omote.
When tori is even slower and uke's balance is not broken, you might have to go for ura (depends on the energy of uke).
If tori does not have any clue about the pre-emptive strike and is waiting for the first strike of uke, he can expect something else then shomenuchi.
Just my thoughts on the subject
preemptive strike is a more advance approach which I am in favor. tori/nage initiated the strike. uke used standard karate respond with high raising block and a punch. tori/nage dropped the ikkyo omote or ura (depends on tori/nage mood) on uke. if uke doesn't raise his/her arm up to block, the shomen uchi strike becomes a palm strike (similar to iron palm) to uke nose. of course uke can always respond differently with a kick or something else, but tori/nage has the initiative and would follow through with something else. If you watched Mary Heiny sensei, then you will see that she didn't wait for uke to start.
sometimes you have to put yourself into a vulnerable position in order to illicit a respond from the other person. Not everything is as it seemed.