Wow. I don't like the Niki much, but stroker and urchin above are really giving it a terrible rap.
As to weapon power, look at the fighter weapon rounds comparison chart at netaces.org. Using the .50 cal hitting power as base 1, the each of the Niki's Type 99 Mk 2 has a relative hitting power of 3.297222. For all four together, we will roughly say that the relative hitting power is 13.
This is FAR more powerful than the 2x Hispano and 4x Browning combination. Roughly, and the numbers are there on the chart if anyone wants to do more exact math, but roughly the combined relative hitting power of 2x Hispano and 4x Browning is 9.
The ballistics of the Type 99 Mk2 also cannot be described as "horrid". They are poorer than hispano's, yes, but far superior to German or Russian 20mm.
As to roll rate, yes admittedly it is terrible, but rudder can help you kick into a roll very well in a Niki.
The Niki has a very good climb rate, and can hang in the vert longer than people will expect. It is a little tougher than Spits, and a large advantage it has over the Ki84 is in the dive.
No, it's not an absolutely superb plane, but it's a little better than you guys above make it out to be.