I even prefer to feed my frogs on a flat surface when using tongs. Placing the frog in a feeding container is hardly stressful at all, and that stress will diminish as the frog gets older and used to the routine.
While none of my frogs sit around eating their moss, there is the off chance that they may consume a small piece while shedding if it sticks to their skin, which will generally pass right through the digestive system of a healthy frog. Its more of a problem with young frogs. Yes, even coco fiber can cause impaction, and Ive seen it first hand.
I do not believe top soil presents less of an impaction risk than coco fiber, as it oftend contains small rocks, sand, small chunks of bark and other things (depending on grade).
Its safe to say that out of all the "naturalistic" substrates, coco fiber is certainly the safest.