I've been flying it a lot. I've been pretty successful. It's my #1 choice now for pure fighter, and attacker unless I'm going into a one vs many attack at a far off enemy base. In that case it's too slow to survive the inevitibale horde of LA-7s and 109G10s that will give chase.
In answer to your question I think there are two reasons:
Gauges
Flaps
1) As others have said, the gauges are awful. I have started threads on this subject before. I'll say it again, the gauges are nearly unreadable for me. Charcoal grey backgrounds with slightly whiter needles, fuzzy, miniscule fonts, upside down altimeter....it goes on and on. It has been suggested that HTC allow or create a custom skin for the cockpit guages, that way the realism crowd can have their gauges and the rest of us can see them!
My less than perfect solution: I don't look at the gauges hardly ever anymore. I keep the E6B always out on my clipboard and have the clipboard toggled to a button on my JS. The only two things I really care about are Speed and Fuel, both of which are on the E6B. The Altimeter is still a problem though.
2) The KI-84 is a great plane above 225 and below 150. Between 150 and 225 you are in the soon to be dead zone. The reason is all the US planes have those great flaps that give very good turn ability from 250 down. And the Spit doesn't need flaps to turn great at 175. However at 175 the KI-84 turns like a dog. It trys to snap roll, it's unstable. At 150 the flaps come out and now you're cooking with gas! I'll stall fight with anything once I get those darn flaps to go out.
However if you pick up a bit of speed....Autoretract on flaps! Instability hits and you have to be careful to not snap roll. OIO can relate to this complaint......
My solution. Fight it as an E fighter most of the time. But I don't back away from any 1v1 fight if they want to make it a stall fight. I just know that when the speed gets to 220 or so, I HAVE to slow the fight to 140 asap! My typical solution is to go nose high spiral climb to scrub off speed. At 150 mph I pop flaps and the fight is mine. I don't think I've lost a 1v1 fight yet once it's gotten down to co-E, flaps out, time. A lot of planes have dove away when they realize what's happening though.
I had some great fights with Pellik last week in his P-38. The Ki-84 vs P-38 fight is a classic, both in terms of flight models and in terms of the real life combats from WWII. They were some of the best fights I've had in the last year.
Now a question for the KI-84 real history folks:
Was the real KI-84 unable to deploy flaps above 150? When you look at them, the first notch looks like high speed fowler flaps on a P-51, or at least a P-38. You'd think the first notch could deploy just like a P-38 at moderate speeds. If they could, then the KI-84 would be one great ride!