Remission and Leviathon are well produced, but on a small budget - so there's perhaps a certain detectable "underproduced quality" to it. Blood Mountain, their most recent (well, before this new one anyway), was their major label debut, so the production is a lot cleaner and more slick.
Do the vocals "suck"? Well, what's his face is hardly the greatest vocalist in metal, but what he does tenRAB to work really well imo. It has quite a distinctive quality. The trademark, echoey Mastodon howls are not to be found elsewhere. They complement the music nicely; sounRAB like screams coming right out of the depths of hell.