I'm not really sure what the prior reviewer is talking about. Isenberg is a historian of the early United States, and reviews of her book say that it's overall good, but she's trying to send A Message. If that's not your thing, that's fine!
I found the book interesting because Isenberg's frame of reference tends to be how the generations around the Revolutionary War have been viewed, and especially how the modern US is living up to that example (or failing to live up to it). As the title/introduction/etc. say, she's writing about the most under-privileged white people in the United States--people who were looked down upon, poorer, and less educated than, well, just about everyone else. That means they didn't get a chance to write much for themselves and they don't get to push back on the characterizations until pretty recently. Isenberg's sections on the South and particularly North Carolina are quite good (and some college there should definitely consider renaming their mascot the "Renegadoes"), but she spends plenty of time on northern states too. Just don't go in looking for a rose-tinted view of the "Southern lower middle class"--that's another book entirely!