If Agincourt is any example, the Roman army would slaughter a medieval army.
5,000 Englishman slaughtered 30,000 Frenchmen. Unless the equipment makes one side hopelessly outclassed, discipline and training will beat nobility and rabble every time. The average medieval army was armed with similar weapons to the old roman armies. Ok, some of the stuff had been upgraded, but nothing the medieval army had would have come as a shock to the Roman army. I.e., it would not be like a cohort of the Roman army meeting a Panzer battalion. It would be more like a Panzer battalion of Mk IVs meeting a battalion of T-34s. A tough fight, but not a problem that better discipline and training could not overcome.
Remember, the Medieval Knight was a member of the royalty and a well trained individual fighter ingrained with the one on one honor bit. Put a bunch of them together and they still fought as individuals. And the footmen, men at arms, archers et all, were considered scum by the royalty. And, if I remember my history accurately, the medieval armies were not very large (too expensive), and it was not until Napoleon that armies reached 100,000 men or more, again. Hail Caesar!