I would also add that in the middle and late empire, many Germans had served in the legions and went back to their peoples bringing alot of skills and knowledge with them. Many of their kings also fought with the Romans as allies. By the late empire, a good chunk of the field army were Germans in Roman service.

But /u/FFS4 is correct, generally we don't see many victories from straight up battles. 'Germany' was never conquered as the land couldn't justify the cost. Contrast this with Dacia which had good farmland and mineral resources, it got conquered over two wars in the early 2nd CE.

At least in Germany itself, the dense forest made it very difficult to move large formations of soldiers around. So, the terrain lended itself towards a guerilla style of warfare that the Germans were quite adept at. The obvious example of this is the Battle of the Teutoberg Forest in 9 A.D where three Roman legions as well as their accompanying auxiliaries were wiped out over several days in the dense German forest. The terrain hindered Roman movement and Arminius, the German leader, who was trained in Roman tactics, was very good at using said terrain to lead the Romans into traps.