Let's suppose I'm not talking about Hispanos.
Let's suppose I'm talking about MG151/20, MK108, and whatever cannons come loaded on the 205.
Let's pretend for a minute that while my hit % is low, I still hit what I aim for on a plane.
In my experience the only planes that have survived a direct 30mm hit to a vital part of the airplane (wing or tail structure), it went down. In my experience, with all planes, a sustained burst to the wing, engine, or tail will produce either a dead or disabled plane.
You are simply aiming for the wrong area, don't aim for the plane- aim for a part of the plane you know isn't heavily armored.
In the case of the Il-2, it's natural for it to fall apart if you hit it from above or behind. It's only heavily armored to the front and bottom. The radiator is still exposed though, and that was the way to down Il-2s after rear gunners were added in WWII.
-SW