One world government would, unavoidably, entail the complete and total erasure of all cultures, gradual stripping of most freedoms, and corruption on an unimaginable scale, all shielded by a mantra of "it's better than there being a war". These things stem from administrative needs, the tendency of bureaucracy to centralize power unto itself, and the lack of external competitors, and all can already be seen to a lesser degree today. Any sci-fi featuring one world government yet not the aforementioned issues thus presents a naive future that literally cannot happen. And before you accuse me of nitpicking, this is just one case among many, as mentioned in my prior post.
Fantasy, on the other hand, runs much lesser danger of running into such issues:
1. writers don't need to theoretize about what will be and can instead simply look to the past (that's not to say they don't come up with retarded anachronisms and magic-related plotholes, but it tends to be less frequent than with sci-fi)