Joker: The Ki-61 is a 1942 design. The Ki-84 is a 1944 design. The N1k2 is the latest design the game has in the planeset, I think.
So you don't think they'd have 1944 designed planes in a 1944 setup? They built 3500 Ki-84s in about a year and a half (less maybe, I don't know exact dates).
While there were quality standards dropping, some of these are modeled in (note we don't have the super-boosted Ki-84 like UbiSoft does? We have modeled the IJA fuel quality rather than US fuel quality).
As for fuel, they had so much fuel they could test jet engines, they could develop and produce many thousands of late-war fighters, put them into action, and STILL field countless kamikaze planes (which still need fuel) as well as nonstop home defense sorties against the incoming US bombers. Fuel wasn't an issue.
You may feel the planeset is unbalanced. I disagree. Given the choice, the US rides are better than Japanese ones. Don't get me wrong I love the Ki-84, but when an F6F dives past it at 500mph (from 20k to the deck) there's no way a Ki84 can follow. Also the Ki61 is exceedingly slow at all alts and sucks above 15k. The Ki84 has a terrible drop in power (and speed/climb) between 8k and 18k (that's a pretty huge band, I tellyawhut). While the N1k2 holds up in a dive better it has an even worse power dip on this same band.
On the other hand, the US planes have magical flaps (in the F4u), speed brake gears, heavy ord capabilities, much better high alt performance, almost as good climb rate (F6F is only 500fpm behind Ki84 climb rate), turn amazingly well at high speeds, dive superbly at very high speeds, can sustain many solid hits with no damage, can get a lot more kills for the ammunition supplied, and when flown right have a bit of an edge over the axis planes: speed.
I don't disagree that perhaps the setup needs to be changed. However it's not because of the planes used.
P.S. We had many folks in A6M5s, which are obsolete compared to Hellcats and Corsairs. Ki61s are nice to fly but slow as molasses and have very small ammo levels. We had our fair share of handicaps.