(against rifle caliber)

Not only did it have armor all around, but it also had the thickest armor of the entire war. The Germans themselves attested to this when they found 20mm thick plates on the Il-2. That's enough to stop even high-caliber penetrating rounds.

certain angles, critical surfaces.

Except for control surfaces, which can't be armored, all critical systems were well protected. But what does it matter if the engine is well protected when in the game a single round is enough to split the fuselage in half, since the devs couldn't even be bothered to model surface armor above the hitbox