One of the problems with — or maybe better to say attributes of — SF&F is the fact that the world of the story is entirely your construction. The upside to this is you can construct the world that best suits the story you want to tell; the downside is that you can also cheat like crazy and get away with it.
But, hey. You're the author, and you have final say over what goes, so everyone else's grousing about what you've pulled out of your hip pocket is just that, grousing. Right?
See, I don't think for a minute authors actually believe that. Just as I have never met an author who set out to write a deliberately bad story, I have never met an author who did not feel she was completely justified in assembling a story in a given way, even if it amounts to a kind of legal cheating.
I suppose what I'm talking about is not even "cheating", exactly — something like where an author deliberately withholds information about the construction of his universe, and exploits that for the sake of creating drama or tension or what have you. I'm thinking more about what happens when someone constructs things a certain way and then lets their own unexamined assumptions about the implications of such construction dictate the path of the story.
The most common example of this sort of thing is in fantasy settings that have magic. The very presence of magic in any setting is problematic, because the way magic can work in a given setting often follows two paths: a) It's borrowed wholesale from other fantasy stories, or b) there's a magic system, one designed with a great deal attention to detail in much the same way as an RPG. Author X's story has a magic system based on weaving threads while Author Y's magic is about mixing drinks. You get the idea.
The danger in either case is how an author can put magic into a setting without thinking once about how anything that normative is going to completely disrupt the sort of rustic medieval town-and-country peasant life so typically depicted in such stories. We've had digital technology for barely half a century, and look what came of that. Given human nature as a whole, I have a hard time believing on the one hand magic could exist for centuries or millennia, and yet on the other hand we'd still be stuck on the level of the draft horse and the water-wheel.
So why do such storytelling elements persist? The legacy weight of storytelling in that vein, which is then mined for making easy connections to the reader's understanding about such stories. It's just easier to reach back into the collective unconscious and pull out pieces like that — to perform "storytelling by implication", as it were. It saves you the trouble of having to think about what you're actually doing.
Note that SF&F are not alone in this. Most any genre fiction can be hamstrung this way, each in its own peculiar fashion; it's just that SF&F get caught up in it that much more because more of what it talks about is supposed to be built from scratch, and the temptation to just fake it becomes that much greater.
Most such chicanery can be alleviated by this guideline: Set up the rules early on and follow them to the bitter end. If something does not work a given way, it's generally poor form to have the major plot twist of the story be that it can work that way, it was just that everyone was too stupid / shortsighted / unimaginative to see how it was possible, etc. — and leave the reader out of it in the bargain.
My way of handling something like this when it came up in Flight of the Vajra is to have that very conflict be placed front and center — to own up to the fact that with regard to Crucial Story Element X, there might be a better and highly disruptive way of doing X, and that if it did happen, a lot of things in-universe would fall to pieces. Rather than try to make an end run around the problem, I decided to try and make the very presence of that problem into part of what the characters themselves deal with, to make the very precariousness of such things into the story itself.
I leave it to you, once the book is actually in your hands, to determine if I did that right or not.