One of the problems with our current systems is that ground is dependent on the shield connection to the engine. If it is not secure there will be a spark external of the engine. Normally everything is contained in a steel pocket. But an external spark is unshielded.
Our old magneto engines had nothing but a resistor plug and an unshielded insulated wire. The magneto was internally grounded in the engine.Unfortunately there is no resistor plug for our little engines so the resistor is included in the cap.
One way to forestall problems is solder a wire to the shield and ground it to the engine. That way a loose cap is not a serious problem