The Roman Empire was the leading force of technology and culture in the Ancient world. Still, relying on the general narrative, we also get the idea that they were quite corrupt as well. The Germans were savage and primitive, for the most part, but are perceived as less corrupt. Is it possible that there were Roman citizens or high-ranking officials who saw through the flashy facade of the Empire and despised it for its corruption, going as far as secretly supporting the Germans?
I think you're reading too much into Tacitus' Germania. His Germans are a fairytale, imaginary sort. Germans were "barbarians" in the sense that they didn't live in cities of the Greek or Roman kind, but that's not all the were.
Arminius, the German hero who destroyed Varus' legions at the Teutoburg forest, was actually a Roman citizen and a Roman-educated military commander. He presents a paradox for your question - here's this urban, educated Roman citizen who is also a Germanic chieftain responsible for one of Rome's greatest defeats.