I often wonder if the Huns, for example, would have been nearly as successful had they attacked Rome during the reign of Augustus instead of 400 years later. Was it Rome's internal problems that weakened the empire? Or were the later barbarian invaders actually just superior to the earlier ones? (I suppose both could be a possible answer too.)
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As the fundamentals of both Roman and Germanic warfare remained unchanged in Late Antiquity, it is difficult to explain barbarian successes is strictly military terms. Germanic society remained politically fragmented. Although Roman authors will speak of "the Alamanni" or "the Franks", these groups possessed no cohesive identity or organization. Actual political power continued to be exercised by petty kings and local aristocrats, who ruled over ever-shifting territorial domains. Large-scale political unity was rare, evidenced by the fact that most Germanic raids were conducted only by a single canton or group of cantons (most raids were thus only ever a few hundred strong maximum). Although Germanic arms and armor were qualitatively equal to those of the Romans, they lacked the political organization necessary to produce arms on a large scale. Mail, helmets, and swords were thus restricted to the ruling nobles and their armed retainers, who were themselves the only "professional" fighters, with most warriors being levied free men.
In contrast, throughout Late Antiquity the Romans continued to enjoy their traditional advantages in equipment, training, and logistics. The elaborate system of state-run arsenals (fabricae) produced sufficient quantities of equipment to arm the majority of soldiers with mail corselet, helmet, shield, spear, and (in theory) sword. Both the field armies (commitatenses) and the frontier troops (limitanei) remained thoroughly professionalized throughout the fourth century, and in the East well until the Arab Invasions of the seventh century. As a permanent force staffed by long-service professionals, the Romans continued to enjoy advantages in training and organization. The Germanic peoples certainly understood this, as large-scale pitched battles were rare. Rather, Late Antique warfare is characterized by smaller engagements, as the barbarians sought to use surprise and terrain to attempt to negate these Roman advantages.
The Germanic peoples of the fourth and fifth centuries thus had not overcome their historical disadvantages against the Romans. I would therefore argue (and agree with scholars who do) that barbarian successes had much more to do with Roman weakness than Germanic strength. Compared to the Principate, the Roman state of Late Antiquity suffered from more frequent bouts of internal political strife. Civil wars sapped the military resources of the Empire in a number of ways, and while all are important, I would most emphasize dramatic losses in trained soldiers and the weakening of the frontiers as relevant.
The advantages the Romans enjoyed when fighting foreign opponents disappeared when they fought each other. Equally matched in training, equipment, and logistical sophistication, numerical strength took on a much greater importance. Emperors and would-be emperors focused on amassing armies of the greatest possible size, resulting in the withdrawal of forces from the frontiers. These weakened frontiers naturally provided more inviting targets, and raids tended to increase in severity during periods of civil war, as barbarian successes encouraged further raids. A debilitating cycle of raids thus occurred, at times resulting in the essential collapse of the frontier. During the civil war between Constantius II and Magnetius in the early 350s, the Gallic frontier collapsed as the severely weakened defenses were overrun by Frankish and Alamannic warriors. Libanius also claims the Alamnni were invited to attack by letters from Constantius. I don't believe this is corroborated by Ammianius, but it nonetheless illustrates how even the integrity of the frontiers could be willingly compromised by one side in a civil war.
Civil war battles also tended to be bloodier, for the reasons mentioned above. Political imperatives often forced decisive battles, further amplifying casualties. At Mursa Major in 351, Constantius II and Magnetius suffered around 40,000 casualties; Theodosius suffered similar at the Frigidus in 394. Many casualties would have been trained soldiers, which took years to replace. Even if buoyed by new recruits (though such heavy losses strained manpower resources), regiments returning to their frontier posts could have been devoid of their most seasoned veterans, leaving them less effective against the barbarians. As we see, a number of new emperors were forced to immediately campaign along the Rhine and Danube frontiers to restore order following civil wars.
The Romans continued to enjoy a number of important advantages over the Germanic peoples during Late Antiquity. These allowed them to win the majority of their engagements, to maintain the integrity of the frontier, and to project power beyond it. Yet frequent bouts of civil war undermined the army's effectiveness, straining it's resources, distracting it's political leadership, and killing it's best men. Obviously the role of the barbarians in the collapse of the Western Empire is complicated, but I would emphasize that, at least in the fourth century, the Romans continued to have the upper hand.