I've been flying the Ki-84 mostly this month. It is a beautiful, fun aircraft to fly and I will find it VERY hard to go back to flying a Spitfire after the scenario is done and that is saying something.
The Ki-84 is so underestimated by the AH majority. No way it should be 20 ENY! it is like a cross between a Spit 16 and a 109 K4, It has one of the best climb rates in Aces High and I would say it is a better overall plane than the Niki.
It's biggest downfall is the high speed area and diving capabilities, to master it you need to never go fast, just bait the enemy to come up to you and climb them into submission. If you lock the flaps in earlier and keep the fight in the vertical it can outturn almost anything. I've found myself beating bad brewster and zero pilots if they have too much Energy and do not control it well enough.
It is also a tough bird for being Japanese, the MG'S are 2 x 50cal size so they do pack a punch if you can get up close and personal around the 200 mark I have made kills with just a few of these.