Atemi is a strike meant to compromise an attacker's balance. Many aikidodoka (aikido practitioners) are uncomfortable with striking, some remarkably so.
There seem to be two primary sources for hesitance to use atemi, at least that I have noticed first hand:
1) They believe strikes are meant to cause pain and injury, which is contrary to the principles of aikido as laid down by the founder of aikido.
2) Mature and civilized people have respect for others and are not violent people. They equate striking with savagery and are actively resistant to indulge in it.