This is an excellent question, Paula.
I have rarely seen any aikido instructors or students who do not have a problem with koshinage.
I think the biggest problem (and the most dangerous) is that students are seldom taught how to control uke's fall by holding and pulling-up his arm. The result is usually that uke is left hanging in the air on his own, trying to contort his body to try to land on his side instead of plainly on his back.
I think many aikido instructors (and students) would greatly benefit from a crash course in Judo before they even attempt any form of koshinage.