MOYG

It has to do with the type of gas that is produced.
I do not understand this statement. If the electrolytic compounds added to water play no part in the reaction at the electrodes, you only get oxygen and hydrogen produced, so what do you mean by 'type of gas'?

I thought that the EFIE was designed to solve these problems - why make life more difficult for yourself by not employing such a device?

We're lucky in that our engines are from 1970's and as such we don't have to worry about, ECUs and fuel sensors and fancy electronics. But I'm sure I'd be looking at using an EFIE if we needed to fool the sensors in a modern engine.