I’ve never experienced anything truly terrifying with the board. I think that it could happen, but probably not EVERY TIME someone uses it, or they wouldn’t sell them so casually. Think of it this way: it’s like reaching into a trash bag, pulling out things and tossing them in the microwave. Nine times out of ten nothing happens. Sometimes you get sparks and burn the house down.
For my part, whenever I used it I tended to notice a lot of dime to quarter-sized balls of light that would flit around when the lights were turned out. I would see these in rooms with blackout curtains, and would not be the only one. My fiance and roommate would see them, too. Typically (for the universe loves to make us look like idiots) we could never get them to record or show up in photos without the WHOLE PICTURE looking like a blurry mess.
There also seemed to be an increase in sightings of Shadow People around this time.