Well put Archie.
If we want to be really technical about it, the main difference is surface area. Edge of hand is to knife as Hammer fist is to, well…hammer.
The more force you put into a smaller area, the deeper the penetration. The EOH tends to get into places a little better than the hammer fist – like the throat, neck and between the legs.
There is no doubt that there is risk of damaging the fingers – but be advised, when used in the edge of hand the pinkie and ring finger will move and not take the direct force of the blow making them more likely to dislocate than break. Plus even if they are damaged, you can still grip a weapon with the remaining three fingers. contrary would happen if you struck with a fore fist where the first two knuckles on the finger will break.
Train them both. Personally I have hit a TON of “things” and haven’t had a problem. But I’ve also trained a great deal of people who preferred the hammer fist in the beginning and either stayed with it, or switched over to the edge of hand after they were conditioned.