I fly the Ki-84 almost all the time in the MA and my observations are all based on experience and facts. 6 points.
1. It has a short ammo count, so shots taken usually need to be on target - and never wasted to get someone to turn. If they're shooting, a good Ki-84 pilot will only fire if they have a shot. If they're shooting crazy all the time, it's a NOOB and you may have the upper hand.

2. It's acceleration is sports-car like - don't expect to outrun it if you're slow and decide to extend out even in a Dora/K4/Tempest as it will catch up and chew you up before it's WEP runs out.
3. It's WEP is very powerful, short, but also recharges very fast. See point 2.
4. And it turns like a beast. If it can't out turn with you, it'll can probably out climb or out accelerate your aircraft.
5. Coupled with a slow stall speed, the thing is like an Oscar with a giant engine and better armour. Don't get low and slow with it.
6. Lastly, always keep your speed up when fighting the Ki-84. It turns best at around 250MPH - speeds above 300 it starts to get a little sluggish. Use high speed turns and engagement to your advantage.
It's like a hornet... you see it, you want to smack it, but your experience tells you...just leave it alone.