The reason it's so dangerous is that the opponent's head needs to be firmly tucked between the attacker's legs or you risk severe damage to the opponent's neck and spine. Just ask Stone Cold Austin.
It was Summerslam 1997 and Stone Cold was to go one on one with the late great Owen hart. During the match, Hart delivered the piledriver to Austin and did not sufficiently tuck Austin's head in.
The impact as Austin's head was spiked into the mat broke his neck and temporarily paralysed him, it was a miracle that he managed to roll Owen up and pin him to finish the match.
The incident significantly shortened Austin's career and from that point onwards, the WWE banned almost everybody from performing it.
It was a standout surprise then, during a particularly incredible match between CM Punk and John Cena on Raw when Punk busted it out, shocking even the announce team.
Punk's expression after the move says it all: everything but the kitchen sink.