I mean that every target presents some kind of resistance, be it a wooden door or a human body or a pile of bricks. I believe the amount of power put in a strike is proportional to the amount of resistance the attacker expects to receive. When the encountered resistance is nil, there must be some kind of loss of balance even by the most experienced martial artists.
A Karateka can hit the air while performing a Kata with great power without loosing balance because he did not expect resistance, but the same Karateka if he's trying to break a pile of bricks will surely loose his balance if the bricks were to suddenly disappear just before the moment of impact.
But I could be wrong.
I take the opposite view. A good martial artist will have no loss of balance on any atemi, strike or action. When I hear people talk about "no resistance in Aikido", I actually view it as what you define above. That there is no internal resistance inside oneself. Which means, that when you go to hit something, you do not build up resistance inside in proportion to expectations. That's what I take the meaning of "no resistance in Aikido" to mean.
And from personal experience, I have felt what it's like to get hit by someone who doesn't lose their balance whether an object is there or not. On a not so personal experience, I imagine that what Ushiro sensei does in karate is a lot like hitting without losing balance or having resistance built up internally.
I veiw good atemi as a disruption to another person's structure without oneself losing structure.
Mediocre atemi is a disruption to another person's structure but with oneself losing some aspect of structure.
Bad atemi is no disruption to another person's structure and oneself losing some structure.