Let me preface this with an apology to anyone who is using this particular trope. It certainly can be done well, and there's nothing (necessarily) intrinsically wrong with it - it simply makes my teeth gnash. No one said personal preferences had to be rational!
The trope that drives me nuts - and I see it both in fiction and in computer games - is the fantasy setting with magic, fantastic beasts, etc, that turns out to be a planet colonized by people from earth. This is either a surprise reveal later on, or is hinted at / discussed throughout the source material with anachronistic references, particularly a specific pop culture element, taken out of context, that we're supposed to recognize.
Gah! I just can't stand this. I feel it's overdone, and it goes against what I feel a fantasy setting should be: another world, disconnected from our reality. (I have no problem with worlds where the fantasy setting lies on a parallel plane / reality.) And if you can explain those elements through science, then it stops being fantasy and becomes science fiction. Even if you can explain some but not all of the world, it blurs the line. (And again, there's nothing wrong with the line being blurred, I just don't care for this way of doing it.)
There's where Pern kind of falls into this and doesn't - McCaffrey is pretty direct about her setting being SF from early on, as I recall. (Correct me if I'm wrong, for it's been well over a decade since I read any Pern.) And I don't mind it so much in Wizardry 8 - which is a lovely little game - because it's bam-upfront about it with fantasy characters being loaded onto a spaceship and marveling at how it sounds like roaring dragons.
I suppose maybe what I'm saying is I don't care for the bait-and-switch. If you're going to give us fantasy-as-an-SF-world, be honest about it. Put the label on the package. Or make the reveal something twisty, mind-blowing and important to the plot.
What do y'all think?