A lone B-17 was not the threat that dozens or hundreds of B-17 posed. Makes sense that a single He-11 or multiple He-11’s in the same type of situation would be of a similar risk.
Also depending on what you’re going after it with, you usually don’t have an armament designed for taking down bombers. Yes 8 .50’s will do some work, and 8 .303’s will shred an engine. Neither will have near the effect as 20mm and 30mm shells. This means you need a longer time on the attack path guns firing to have the same effect. If you are in a Spit/Hurricane/Mustang/etc (anything with an inline liquid cooled engine) you don’t really have a lot of durability to handle hits to the front of the aircraft. Radials like the P-47 are more durable, and you might find more success.
As @wheelsup_cavu mentioned a formation of bombers makes life tricky. Use the same taffics the Luftwaffe used and you’ll probably see reasonable success.
Try it with a P-47, and try it with a Mustang. They both work, but one is a lot more likely to get you home every time.
I believe the Germans had added the cannon’s following the Spanish Civil War based on lessons learned about the effectiveness of machine guns at shooting to aircraft. It was one the British didn’t learn until after the BOB.