I have taken numerous hits from Hispano's with no loss to my aircraft, most recently from a Typhoon to my Ki-84's wing.
If I am being inaccurate I will get two or three kills with the Ki-84, but if I am on target the gun package annilates fighters in very short order.
For example, one of my last fights was against a couple of N1K2's and I spent 284 rounds of 20mm ammo getting an assist on the first one and the remaining 16 rounds completely removing the second N1K2's wings, tail and setting it ablaze.
The difference? Accuracy. If you hit with the Ki-84, it will kill.
One of the big reasons it is unpopular is that, coupled with it's merely average firepower, is the fact that Combat Trim cripples it's manueverability and most people, I suspect, use Combat Trim.
I don't have more info on the guns right now. I can post some from Tony William's book when I get home though.