Even before TFH, aircraft were very potent. They don't instantly kill enemy formations, but nothing in the game can withstand weeks of bombing. TACs in particular are devastating when used properly against softer targets, and CAS shred armored formations if left unchecked.
In TFH, aircraft got even stronger because they always penetrate no matter what tech the planes and tanks have. To test it, I ran a game as the Soviets and just built INF/ART for ground forces and built an air force that was simply huge. During the initial invasion, the Germans seemed to advance unchecked, and then we they got close to Leningrad at around the 6 week mark. And then they stopped and collapsed completely. Even under Theater AI control, my aircraft were darkening the skies and never stopped bombing them, so by the time the war had got to 6 weeks, half the German army was out of ORG. Units that lost ORG in land battles never regained it because bombers would hammer them back to nothing. Casualties weren't too severe, but who cares if all that fancy German armor has no ORG?
It took 18 months to get to force surrender because INF/ART by itself isn't ideal for encirclements or fast penetration. We had to assault along a broad front and theater AI was being cautious because it saw a ton of German stuff and didn't feel like blindly charging in.
But in the hands of a human, aircraft are even more deadly because no matter where my opponent masses armor for an offensive, if I have air superiority, I can just hammer their armor until the armored spearhead is a plowshare. And that's not even counting logistical bombing...
In the hands of a human, aircraft are even more deadly