Santiano is not universally loved. They're too close to Schlager for that. And the issue with Rammstein isn't really alleged nazism (which only came up briefly when they used Leni Riefenstahl's footage) but rather their singer's alleged backstage raping.
So far, I haven't met people who hated Herbert Grönemeyer, Marius-Müller Westernhagen, Jan Delay, Nina Hagen, die Ärzte, die fantastischen Vier or Reinhard Mey. A little more recent music that I have rarely seen associated negatively would be by Kraftklub, Peter Fox or Deichkind. None of this is particularly progressive, but it's decent stuff that is so universally liked that e.g. weddings often rely on them to get people to dance.