It's the longe range gunnery aspect.
Outzooming Spits with a 300~400 yards margin is easy.
Problem is, in every given situation the 190s must prepare excessive Energy that will alow it too zoom with a margin wider than at least 600 yards. Anything under that, and any body can snipe everything out of the sky, as long as he is willing to pull the trigger down long enough.
Another problem is the way Spits or N1K2s stall out, while compared to how the other planes stall out. Even with manual trim, a straight vertical to 0mph will often cause a inverted flat spin. In the case of the Spit or N1K2, almost no such problem. Since there is no fear of it, their aim remains rock solid during the vertical.
Another interesting thing is how the torque forces work in the Spit9 or the N1K. I'm mentioning this because, the other planes show very different flight characteristics between combat trim and manual trim. However, when I fly Spits, N1K2s or such, I keep get confused whether I'm flying with MT, or CT - can't tell the difference. The only indicator to me, is the nose pitch-up movement when speeds increase, in MT.
Ofcourse, all empirical feelings, nothing more.
But I think there are valid reasons that the Spit9 or Spit5, maybe inevitably, are better than they should be. Of course, that don't necessarily mean the reasons are exactly as Mandoble gave.
