The Germans of the Migration Periode certainly had improved a lot. That's the result of centuries of intense contact with the Romans, even for those that lived far away from the Rhine. After having given up the idea of conquering Germany the Romans started supporting certain warlords with arms and money. These in return became powerfull enough to subdue other chiefs that way, becoming even more powerful, the Franks would be a good example.
Adding what Arjos said, that the Roman army of the late periode was in large parts composed of German mercenaries itself, we would not have much difference between a Roman army, a Frankish or one of Goths. In fact in many occasion, like Catalaunian Plains, "Roman" battles in the late peropde rather were battles between oposing German groups ("tribes" would have been the wrong term to describe the new kingdoms).
But that of course was four centuries after Ariovist or Arminius. Neverthless I think it is fair to assume that there had been a larger group of professional warriors in Germany, and those certainly would not have been satisfied with primitive shields and simple spears, weapons that would had given them no advantage over the common peasant - and weapons with which they better not had dared to show up in Gaul.
On the other hand, there is absolutly no hint that the Romans underestimated the Germans: the first time they encountered those people some 20,000 Romans went down in Noreia, the second time a good 80,000 Romans were butchered at Arausio. That war the Romans only won by producing a military genius and overthrowing their entire material consitution (what in the long run meant they had to sacrifice the republic to get past the crisis).
Next time they encountered Germans was when Ariovist was mopping up the Gauls and in the process of establishing a German kingdom in France. And then there was Teutoburg, a battle what caused nothing but sheer panic in Rome and resulted in a campaign of not less than 1/3 of the Roman in Germany - without any result...
Bookmarks