Post film. A bomber is a big aircraft and has a lot of area to take hits, and the B-26 is noted for its durability. The engine is a little surprising, but not impossible depending on where you hit exactly.
As for the rest, I'd be surprised if 4 30mm's all over the airframe took down a bomber. You'd have to get lucky and kill the pilot.
The 30mm's aren't wonder weapons. They don't magic parts off the target. If you think they are, or act as though they are, you're an idiot.
Looking at the statistics at the time of posting, the 109K has killed 1544 enemies, or almost as many as all the other 109's combined. The 262 had killed 1017. The 163 has killed 268.
In 17 days, that's 2829 kills we can confidently attribute to the Mk 108. Or 166 kills per day, 7 per hour (including the almost empty US night time period, more likely around 17 per hour).
Hardly disappointing, and I would say sufficient proof that the 30mm remains an effective weapon.