We did EEG studies at school where I was on both sides as the subject and as a student. (cog sci) Despite that I am not an expert on the methodological problems and challenges this kind of data collection faces or what the implications for experimental results might be.
Your question is a serious one that only a specialist can really answer / debate / investigate and that answer may not be comprehensible to you or to me either.
That said, it’s clearly a job for Google. Wikipedia always has pretty good and getting all the tie better stuff - they have an EEG entry.
Here’s a couple I turned up on the methodological challenges faced by this kind of research.
It’s a good question. You could spend a career answering it. Ditto with fMRI