Mitsubishi Stuka and Thunderbolt

[The_WOZ]

As most planes in the game, both the P-47 and the Fw-190 have complete internal collision boxes modelled. This is, individual models for each internal system with a rough shape and size.

This means that if the ignition system is shot out, is because a bullet indeed hit that small part of the engine.

Now, there's another group of planes, which includes the Stuka and the P-39/P-63, that do not have any internal collision boxes at all.
In these planes the damage to internal systems is determined procedurally every time a bullet hits the airframe.
For the Stuka there's around a 60% chance for a incendiary bullet shot to the wing root to set the wing tanks on fire.

I have once hit a P-47 with 80 20mm rounds (from a J2M) and it flew away. Other times a few hits from a 109 on the wing root will bring it down.
Even on planes with complete damage models there's some randomness thrown in to make things more interesting and realistic.
For example, back to the ignition system in the P-47, a bullet may not have enough force to knock out a magneto, but it may still sever some wiring and have the same effect.

In the case of the F6F, as a Ju88 and Betty pilot I can attest to the engine toughness to both MG and cannon fire.