I'm not an expert. I did some undergraduate courses in neuroscience, and one of those covered how various instruments work, including MRI, EEG, and a few others. I would invite anyone with a better education to correct any errors they can identify in what I've said.

And yes I DID just refresh myself on those WP articles I mentioned :-) and they seemed pretty well written, to me.

I also owned a NeuroSky a while back, and IMHO it was not very useful... But that's because it was a toy, not a medical device. Same underlying measurement principles, but very different in terms of actual operation.

One of the main differences is that medical devices are always attached to bare skin with conductive gel applied under the sensor. Also, medical devices have more sensors. This vastly improves the signal quality, as compared to the toy devices.

In research work, medical EEGs have been successfully controlling computers for several decades, long before NeuroSky or the Necomini ears came to market.

Tl;Dr, you don't need an FMRI to control a computer... EEGs work fine, but none of the existing EEG toys have been particularly well designed in that regard.