Why it does not get used:
* The Ki-84 does not have the cannons and ammo load that the Niki carries.
* It's not particularly fast (although has good accelleration), though Ki-84 pilots find it fast enough for the most part.
* Not a tough plane that can soak up hits, unlike the F6F or Jug.
* It's not the best turner in the game - but can outrun, outclimb, or out accellerate those that it can't out turn.
* It can hang on the prop and a great vertical fighter, but don't get too attached to vertical fight or someone with a lot of E will get you.
But, the Kia 84-Ia is my favorite defensive plane and furballer. I like fighting down on the deck with it (which goes against conventional thinking in holding high E). I'm not a great fitr pilots by any means, but I have my best luck with the Frank compared to rest of plane set.
1 on 1 matches should often go to the Ki-84. Usually it's the second or third enemy plane that gets me. I've even managed to hold off three of four B&Z planes in a furball and get a few hits in.