In fact, well-made modern replica armor is better than the best medieval models.

The quality of medieval steel was extremely variable because of the blister-and-hammering process used to produce it from wrought iron. The larger the piece, the worse the quality control problems.

They could't control the carbon content of large flat pieces of metal, and also there were a lot of slag intrusions in the iron to begin with, which hammering couldn't get out.

The carbon content in turn strongly affects how the metal will respond to quenching and tempering and other heat-treating methods of hardening it and reducing brittleness.

Consistently good steel required crucible casting, a technique unknown outside India in medieval times and rare there; it wasn't introduced into Europe until the 18th century, and then only used for edge tools and springs.

Really consistent face-hardened armor was a rarity, the product of a combination of high skill by the armorer and a rare batch of really good materials.

Ordinary export-quality Milanese plate which has been tested varies from quite good face-hardened steel to what's effectively soft wrought iron. Sometimes a single breastplate will vary that much from spot to spot.

It was much easier to get good quality in a _small_ piece of steel; a sword, a lancehead, or an arrowhead.

In terms of design and construction late-medieval European plate armor was about the ultimate in pre-gunpowder protection, but the quality of the materials left much to be desired.