The answer to that question is the former. The styles fit together well, with the metallic guitars giving even more heft and power to the swinging Southern attitude. The sound works. There are a few places where I think the actual tones selected by the band and producers could have been better, but the general tenor of the metal country fits the music exceptionally well. It sounds natural enough that I'm quite shocked more bands haven't done this more often.
But while the sound of the album is both interesting and fitting, that doesn't cover up the songwriting, which is not nearly as well considered. I don't know if it has to do with trying to have one foot in two different worlds, but the writing on the album just isn't sharp enough. The songs don't have catchy country riffs, or choruses that can carry them along. Both aspects are rather bland, which is something I feel bad saying, since I think they have a good idea going here.
What the band could be is encapsulated in "Stonewall Jackson", which is the one place where the band hits on the possibility they hold. It's got the back-roads attitude you're looking for, but it tempers that with the kind of hook that the best of metal bands would be proud of. If the entire album was able to live up to that standard, we'd have something we could look at as the start of a new and growing genre. But, that's not what this is.
"Swamp Rebel Machine" is a good idea that needs some more work. There's the beginnings of something interesting here, but right now the band isn't able to bring it to fruition.