I don't understand your argument. Comparisons of values across history are not flawed just because they lead to contradictory conclusions. They just run into problems when people attach an artificial level of authority to the targets of comparison in order to avoid having to justify their ideas in the present (ie the founding fathers said...), and typically that's accompanied with sloppy methodology and outdated or incomplete sources anyway. That's also true for making assumptions about ancient values and culture broadly. The sources are complicated to work with, but there's a lot you can do with "obviously biased" accounts and what a couple of guys thought of the world around them that meets a high bar for the degree of certainty relative to the sources, especially when you cross-reference with other fields. It's never final and you'll see that in the way responsible academics state their conclusions, but that's not the same as saying cultural studies and comparisons are inherently worse than other ways of organizing study of ancient history.