Ikkyo, when applied correctly and powerfully, will turn uke heels over head. Unless an "ikkyo breakfall" is taken, this will cause uke's face to either get crunched into his or her knee or, perhaps worse, right into the ground...
Thank you for adding this. I was looking at my prior response and found it lacking much upon further review.
I'm also struck by the causualness in regards to shiho nage in this thread. In my humble opinion it's one of the most dangerous things we do. There are a wealth of opportunities to mess it up and have someone get hurt. Injuries can range from a broken arm, to concussions and possibly death (or so I've heard in third or fourth hand form).
Just to add a bit on injuries. At the dojo where I spend my days a student was taking a kyu test. During the test she leaned a bit too much and uke accidentally kicked her. A day later she collapsed from a stroke caused by a clotted artery from the kick. By all accounts she appeared fine when she left the dojo. She lived but suffered brain damage. My understanding is some speech and coordination was lost.
There are no purely safe techniques.