This is simply not true. On the whole Roman armies were far, far superior to the late horde armies. The problem was that the upkeep on those armies was more than Rome could afford to pay.
And Marian legions are generally considered to have been better trained, organised, and motivated than later legions, which relied far more heavily on conscripts and barbarians. One could even argue that the legionnaires of Augustus' day were better equipped than the late legions- for example the former had lorica segmentata, and the latter did not.
Afraid it is, instead.
Main advantage of the roman army in the I centurt was a better organization .
If you read later historians (e.g. Ammianus Marcellinus), you often read disconcert at seeing "barbarian" armies (goths for example) behaving in an ordered and regulated manner "as if they were trained".
The point is that rough I century tribes had evolved in more complex structures and their army, too: you do not fight 300 years without learning something.
Both in terms of combat efficiency and versatility, IVth century smaller legions (1000 soldiers or so) were much better than Ist century 5000 men Marian phalanxes, but I admit that the Marian ones were cooler.
in term of equipment, too.
Lorica segmentata is the classical error of perspective: you should not think of an ancient sword as lasersabre able to cut through stone and steel, but rather as badly-sharpened cutlass which often was used almost as a club
If most of the blows you have to protect against are smashing rather than cutting in nature, then pressed linen is a better protection than segmented armour (anyway, even if these-days actors tend to wear Lorica segmentata, in roman times it was quite rare: most of the soldiers had Lorica hamata instead)