Additionally, the Roman's ability to play Germans confederations against each other for 200+ years. Which worked for the Roman's fine since the Rhine is a natural border that is more secured than an arbitrary line in the woods.

To add to this, all of the civil wars plus general decline of army discipline really hampered the ability of the Roman's to raise legions and fight them effectively. Additionally, by the 3rd century, it was not uncommon for a general to win a big victory, proclaimed emperor, and fight other Roman's for control.