To put it simply, none of the above is really true. Roman tactics were neither ineffective nor had the Germanic peoples overcome their traditional shortcomings against the Romans in logistics, unit cohesion, and equipment.

It is important to remember that groups such as "the Franks" or "the Saxons" were not cohesive political groups, but rather broader, loosely affiliated confederations of local kings and nobles. It was these lesser rulers (known in Roman sources as regales) whom exercised direct political control over a territory and it's inhabitants.
Large-scale political unity was rare, best evidenced by the fact that most raids were conducted by only a single cantonal group. Germanic socio-political structures were thus highly localized and decentralized.

Unlike the Romans, only a small comportment of any Germanic force would have been (semi) professional. Most warriors were levied free men, under arms only for the duration of an attack or raid. There were no systemized regimens of training, nor would individual warriors have been as comprehensively armed or armored. Although the Germanic peoples could produce arms of high quality, they lacked the organizational sophistication to produce them in sizable quantities. Thus, the standard panoply for a Late Antique Germanic warrior was only a round wooden shield and a thrusting spear, with archaeological evidence suggesting all other equipment was rare, outside of the elite and their retainers. These retainers, known in Latin as comites, were the semi-professional fighters maintained by Germanic elites. They were generally much better equipped (with helmets, swords, and mail common) and were generally better trained, and formed the core of any Germanic force.

By contrast, the Roman army remained a force staffed by long-service professionals. Constant training emphasized discipline, unit cohesion, and battlefield maneuverability. The complex system of state-run arsenals (fabricae), underwritten by the wealth of the Roman world, produced sufficient quantities of arms to equip most soldiers with a mail corselet, helmet, shield, thrusting spear, and (ideally) a sword. The centrality of training to the army's success cannot be emphasized enough. Training, when married with the other factors of Roman military culture (discipline, competition, and intense loyalty to unit and standard) created high levels of unit cohesion; the ability to execute complex maneuvers "under fire". Obviously it would be foolish to imagine that all Late Antique regiments maintained such high standards, as the army suffered it's fair share of lax commanders. Yet on aggregate, the standing regiments of the Roman army were better trained and better equipped than their barbarian opponents.

This deficiency helps to explain several aspects of Germanic warfare; chiefly, the avoidance of pitched battle and the use of shock tactics. Whenever possible, Germanic forces avoided fighting the Romans on open ground on their own terms. Rather, the barbarians sought to ambush the Romans, or fight them on unfavorable terrain (and the two are not mutually exclusive). The intent was to disrupt formations and prevent the effective deployment of cavalry, as well as maximize the effectiveness of the charge. Ideally, a well-placed ambush in advantageous terrain (such as a forest or marshy ground) would place an opposing Roman force at a critical disadvantage.

When arraying for battle, a Germanic force would deploy in relatively close-order, with unit groupings based upon tribal affiliation or shared kinship. At the front of these formations would deploy elites and their retainers (comites). Germanic elites were expected to lead from the front, and their retainers provided a heavily armed (yet small) force to spearhead an attack. While it is entirely possible for these warriors to have "locked shields" (which in itself is a relatively simple tactic to employ), the practice was not common enough to warrant comment by Roman authors.

Led by this professional core of comites, a Germanic force would thus hope to overwhelm and break apart a Roman line, relying upon speed, ferocity, and psychological terror (such as the barritus war-cry). Ideally, a swift charge, (spearheaded by warriors armed and trained to the level of the Romans) could break apart a Roman line in a short period of time. Germanic deficiencies in equipment and training meant that protracted engagements were to be avoided. Thus, the element of surprise and favorable terrain took on a greater importance.

The Romans maintained a number of advantages over the Germanic peoples in Late Antiquity. Chiefly, Roman forces were better trained, better equipped, better supplied, and had a strong cavalry component. Tellingly, these advantages remained relevant, as the Romans continued to win the bulk of their field engagements. Germanic warfare remained characterized by an avoidance of pitched battle and the use of ambush and terrain to disrupt these Roman advantages and maximize the power of elite-lead shock tactics. Remember, the end of the Western Empire is a political, not a military event. It is wrong to view the end of the West in terms of deficiencies in the military system, both because Roman authors themselves did not, and because this system is maintained in the East until the seventh century.