Personally, I've heard accounts of several life-ending and life-threatening injuries from ukemi.
All were related to falling backwards.
In those where the uke actually died, it was from being repeatedly thrown onto the back of their head in shihonage.
In the life-threatening case, it was a broken neck from being fallen upon while demonstrating a rear roll.
Consequently, I try to *never* do any ukemi that places me in a position to smack my head into the mat and I try to *never* outside of ukemi-practice time (that is specifically structured to keep people from falling on each other).