Wilbus, the Ki-84 has been one of my staple rides since it came out. I havent touched it much this tour, since I've been trying the "worthless planes tour", but next month I'll be back to my F4U-1 and Ki-84 standard. The two things that make it difficult for a new pilot to learn (IMO) are:
1) Low speed handling is great, but only if you can get those flaps out. Trying to maneuver at 100mph without the flaps is like falling into a vat of syrup and trying to swim around in it. You really have to either be very familiar with the way it flys, and know when you are slow enough, or train yourself to watch your airspeed. It can also be hard to get it that slow sometimes, once you are committed to the fight. It helps (IMO) to cut the throttle while you have a bit of space, get it slow enough, engage the flaps before having to get really down and dirty in it.
2) Learning a good POA for consistent gunnery is difficult because of the high ROF of the guns. They dont have the ballistics of the .50's say, or the 20mm Hispanos. They ARE very lethal inside 300 yards, once you learn where to point them for best effect. Takes time. This also means that while learning, your guns are going to empty faster because they fire so fast. So not only are the newer pilots frustrated by lack of hits, they run out of ammo too fast and become sitting ducks.