a lot of US law relies on the "reasonable person" standard, and i'm fairly sure this is true for most law systems. this is, of course, super vague. but you can't really construct an objective test for "is this a threat" or "was this consequence foreseeable" (what, are you supposed to evaluate probabilities?)
you want to minimize the vagueness to be sure, especially when it comes to things like "is this speech constitutionally protected or not" (and the Miller test specifically has some criticism: it mentions 'contemporary community standards' but doesn't clarify if the community is the US as a whole, the speaker's community, or what). but you can't really make an objective test. and so the problem with people in power using obscenity laws to oppress queer people isn't that the laws are vague, it's that the people in power are bigots. you can add "being trans isn't obscene" to the law and they'll say "well, it's not being trans, it's the fact that it's PROPAGANDA" or some shit.
the system can limit their ability to do harm, for sure. but if enough of the commissars/senators/people's representatives/whatever are bigots who want to make your life miserable and are willing to bend or break the rules to do so, then there is no system of government that will save you.