I'm not into Daddy/Daughter so I can't really say, but does it feel similar to how people call their partner daddy/mommy to you? Like I can get down with calling my wife "mommy" but she doesn't like it so I don't call her that. But even for me it's just the mommy aspect, her calling me son or a child feels weird. That's not part of it for me.
I mention is because this is a good example of what I was trying to describe in the past. To me, someone saying "I have a daddy kink" or "I want to be called daddy" all feel much less taboo than "I have a daddy/daughter kink". I guess because it feels similar to pedophilia (I'm not saying it is, just trying to work out why I think people would treat it as taboo). That said, as a man being called "daddy" by a woman is basically them saying "I'm your daughter" in a sense but it feels like folks don't want to treat it the same.
Health and safety are definitely a pretty clear line for when fetishes become taboo. Like if someone is into choking that's one thing, but if they want to pass out that feels disturbing.