The receiver unit fits under the padding in the back of my Scorpion 700, and the LED wand goes under the padding as well and comes out to the corner of my right eye. The boom for the wand is relatively small. Not as small as the Autocom stuff I have on the other side, but I don't feel it under the padding.
I have the receiver at the very bottom back of my helmet. I can reach back and turn it on and off. Where it sits is below the occipital area. It's sort of a nook in the back, and I barely feel it. If the receiver is out of place, I can REALLY feel it. My mellon is pretty large and fills the helmet. Your head shape and ability to get the receiver in your helmet comforatbly might vary. I think if it's comfortable, it's safe. If you feel the receiver protruding in any fashion, yes, there's a good chance it could impale you. But it's small, perhaps half the size of a zippo lighter? For the most part I don't even know it's there. Until it lights off.
BTW, I don't have it fastened in any way, it just stays there. Mostly held in place by the helmet padding. Same with my Autocom internals. And the bottom edge of the little hard plastic square box where the battery goes (receiver) sits at the bottom edge of my helmet. It can slip in and out easily until my mellon fills the bucket. Then the receiver has never slipped out (like on a LD run). YMMV.