I mean, I guess I get the fears of worsening divisions, but in specific terms is this really all that different than Man in the High Castle? And I'm not sure where the claims of this show being a 'mess' are coming from, except that people just don't like that it exists at all. Honestly the only thing that bothers me all that much about it is the idea that (I'm assuming this takes places sometime in the 20th century, maybe even modern times) the CSA would still be a fully slave-holding nation despite the massive international pressure to end slavery that the British put on the world in the late 19th century. (Britain being literally vital to the Confederate economy)
It does suck that a show like Underground isn't getting a real chance, though. That's an important subject that doesn't get enough attention and is actually incredibly unique in the tv landscape (not to mention ripe for drama).