Re: Ukemi vs. Protecting your attacker
One of the things that "bad ukemi" can mean is that the attacker is not attacking. Then if you want to go through with a throw that normally redirects the attacker's force, you don't have anything to work with. You can substitute force of your own, or accept failure....
In two years training with her I've only seen my fourth dan teacher completely fail at a technique once. She was trying to throw an unranked student kiri-kaeishi, a throw that involves taking uke's energy and bouncing it back in the direction from which it came. like a ball bouncing against a wall. Only, the ball had already come to a stop by the time it reached her, so the "bounce" just didn't happen. (I tried brute force, to my shame; that didn't work either, as uke was bigger and stronger than me.)
I think that if that student actually went after me to clobber me, I could throw him. I also think he might be surprised how well the ukemi would go; I know I was, in similar circumstances. But when he's thinking (as he said himself) 90% about his ukemi anxieties and 10% about "which foot was supposed to be forward on the attack?" that doesn't leave much intent or impetus to redirect.
That said, there are certainly moves where if you don't know what's coming, you're likely to get hurt. I come from a school which doesn't do kotagaeishi and have had a few close calls when cross-training; if your knees are locked and weight back on your heels when nage goes to turn you over your arm, you may not be able to follow fast enough to save your wrist, no matter how well motivated you are. Perhaps better technique from nage would help, but it seems very difficult to be that good.