I think the impact thing is true in the majority of cases, but like all things, there are exceptions. I've seen some extremely well done impact shots. I would say though, that its hard to do right, and unless its important to the comic's tone or story, you're better off showing post impact.
I think one of the things that allows it to work for the artists I've seen do it, is that they often show speedlines coming out of the other end of the target. Like a punch to the chest producing effects out the victim's back.