Depends on how you fly the plane Fatty. The 190A8 isn't much good in any situation, unless you are attacking low level buffs, then it shines.
Calling a plane "hard" or "easy" to get kills in is basically describing how well the plane fits into the MA. An example of an "easy" plane to get kills in is the N1K2 (it sure isn't so popular because of its historical reputation, agreed?). It is probably one of the best accelerators in the game, one of the best climbers in the game(sustained and zoom), one of the best divers in the game (again, sustained and not, but it is better at the beginning), and one of the best turners in the game (behind the Spits and Zero). On top of that it has 4 20mm cannons with a relatively large ammo load. The only negative thing about the plane is it is pretty slow in level flight.
Since most of the combat in Aces High involves turning, diving, climbing, and accelerating, a plane that does well in these areas will do well in the MA, even if its top speed is comparatively low.
On the other end of the "easy" scale are the very fast planes, with the LA7 being on the top. It is the second fastest plane in the game on the deck, which makes it easy to run away from slower airplanes that have gotten in a good position. Furthermore, the La7 easily outturns all the other "fast" planes (dora, P51, Tiffie), and out accelerates them as well. It has a climb rate equal to the 109G10 up to around 7k, and dives well. I guess you could say that the weaknesses of this airplane are its low speed handling (which doesnt come into play very often), and the ammo and fuel load (both of which are fairly small). The La7 is a "easy" plane because you can kill the other fast planes comparitively easily, and run away from the N1K2 and Spits.
In the middle you have planes that I would consider "challenging", if not hard. For me, this category includes most of the U.S.A planes (with the exception of the P51), most of the German planes (with the exception of the 109G10 and the 190D9 [and the G10 is iffy]), the other Japanese planes and the Italian planes.
I consider them "challenging" because when you take off in them you know you are most likely going to meet planes that can A. Outturn you, or B. Outrun you, or C. both plane types together. In my opinion, this makes it harder to be "successful" in those planes. You may disagree with me, and that is fine. All I am doing is explaining my opinion, and possibly clarifying the opinions of others who may agree with me but haven't expressed it this way.