To me the question is like asking 'are you afraid of crashing your car when you drive (or your jet when you fly
)'. If I am working with someone whose ukemi ability I think might not be up to the technique, I modify it, slow down, or don't throw them. If it is my skill in the technique I question, I slow down. I think the only time I'd have to be afraid would be if I hadn't taken the responsible steps to mitigate risk.