The difference between Napoleonic, victorian, etc. eras vs. slavery and/or WW2 is that we have a subsection of the population that has been very clear that they see themselves as the spiritual successors to the slaveholders AND the Nazis, and that given a chance they will try to re-enact those policies.
People can have fun with historical periods that don't have direct current events tie-ins, but when normal people are confronted with weird fascist dog whistles, it gets very uncomfortable very quickly.