IMHO, the key here is "high fantasy."
In a sword-and-sorcery campaign, I think sci-fi elements fit in fine. The rollicking, anything-goes, don't-explain-it-just-get-to-the-action style of much S&S blends fine with just about any other action genre. While Conan may never have encountered a genuinely "sci-fi" race or entity in the REH stories, I could easily see it happening; even more so for Fafhrd and the Grey Mouser (and what ARE Ningauble and Sheelba anyway if not potential sci-fi aliens?).
For high fantasy: No thank you. I currently run a more or less straight up campaign in FR featuring much intrigue, deep lore, lost magics, and elegiac setting elements, and having a blaster or the like feature (except possibly as the punch-line for what magic truly is in the world, etc.) would disrupt the feel of the game.
Incidentally, I never saw Spelljammer as science fiction, nor did I play it that way. Spelljammer always struck me, with its phlogiston and strange gravity laws and baroque ship design and the like, as being of the Baron Munchausen or Gargantua genre; true space fantasy of a style arguably more antiquated than that of LotR or the like.