“Hard” SF seems to demand rigor in explaining the science–otherwise you get into Fantasy territory. That being said, trying to define the parameters of any genre is difficult at best. Is Star Trek SF or Fantasy? Star Wars?
But yeah, interesting question for those writing specifically to those genres. I’m a “Speculative Fiction” leaning person. If it’s beyond the mundane, and fiction, it’s Spec-fic. I write in too many “genres” to get bogged down in only one, and it seems that most SF-Fantasy-Horror writers tend to bleed over the edges at some point.