No, you can mix fantasy and science fiction elements all you like in a story. It is then a fantasy story with SF elements. Steampunk fantasy, for instance,
Well if I have until the end of Saturday, US time, i might make it...
Anything by David Gemmel would fit the OP request.
I think that's the wrong question. The answer to your question is: Some will and some won't.
The real question is (imho) : Do you think it's believable?
I've been shopping my first manuscript around to a few agents, just getting settled to endure those hours of consciously driving myself mad waiting for