Explanation: In the Principate or the Early Roman Empire - what most people think of when you say 'Roman Empire' - the Roman Legions were a fearsome force. Overwhelmingly made of well-paid citizen volunteers guaranteed a cozy retirement with a mixture of career officers (centurions, prefects, and some tribunes) and political appointments (some tribunes and the legate - the general in charge) at the upper ranks, each Legion was a mostly self-sufficient engine of war that could (partially) supply its own needs and repairs, and even undertake independent campaigning action if absolutely necessary. The Legions maintained a high standard for professionalism and discipline by the standards of the Classical world, and was a feared fighting machine of unmatched efficiency.
By contrast, while the Late Roman Army was facing generally better-equipped foes (at least on the northern borders), it was also a dispirited force of life-conscripts released only when they were no more use to the meat-grinder, reduced to dependency on centralized supply lines, split into penny-packets which were little use if caught on their own, and often flooded with barbarians forced or bribed (or more often, some mixture of the two) into service. The infantry of the Late Roman Army were not incompetent, but lacked the reliability, flexibility, or efficiency of the Legions of old; and the entire broken system of organization and command led to the very manpower base of the Late Roman Army falling apart, leaving the Late Empire eventually reliant on barbarian troops who, while as effective as the barbarian enemies they faced, the Empire did not trust and could not use like the Legions of old.