We all have our limits for the crazy stuff we will, or will not, accept. They are just at different levels or types.
The best fantasy worlds have their own rules and must operate within those rules or the story falls apart - and there are some that do fall apart, I had one series do this around book 5 or 6 (they were long books). And some do cheat for their ending (literally the hand of God in some cases), although a few seem to get away with it, at least for most their audience.
All that said, one of the things I've noticed about the many variations of stories based on ancient mythology is that the rules tend to be quite loose. This can be because my knowledge is limited, and so sometimes the author is using some aspect I was previously not aware of, but I also think it's because the myths themselves played pretty fast and loose. I didn't find Percy to be any more guilty of this than most.
|