Do you bother listening to what the fighters themselves say? They don't mind stomps or kicks to the head, but they highly dislike elbows on the ground.. Not because of damage but because of cuts, especially by the corner of the eyes, which can lead to serious eye issues. Leaving that aside, it's just unnecessary cuts that create scars. All of this is done by short elbows that have little power and are aimed to graze, not hit directly.
I know SOME fighters, can generate good power with elbows on the ground and actively seek to KO the other but most ground elbows are meant to cut and nothing more.