I guess the big difference comes from the fact that for me personally, the head FLYING off doesn't really matter. If it were to just drop off realistically, I still receive the exact same impression of brutality, there's no additional "point" made due to unrealistic crazy flying heads.
For me it kind of sits in the same category as unrealistic sword sheathe sounds. I don't need some obnoxious metallic clank to know I've drawn/sheathed my sword, you could give me just that dull realistic leather slide and the point would be exactly the same. However at the same time, I respect why CDPR might have chosen to do it in this way, it's just the style and I don't particularly have a problem with it.
Everyone is sensitive to different things as to what becomes realistic for them, or what counts as (and breaks) "immersion", and in this case I understand and respect both sides of the argument. Although at the end of the day this is CDPR's creation, and I highly doubt anything is about to change at this point in development (Not to mention I'd say the folk against this are in the extreme small minority - not that it makes your opinion any less valid). So like myself and issues with the Alchemy system I think it's honestly just a matter of acceptance at this point.