My understanding is that the lorica segmentata was a very complex in its design, with numerous plates linked by copper-alloy buckles, hinges, and hooks and a leather harness underneath. It is understandable that all of these little pieces, which appear to have commonly broken and fallen off, would present maintenance problems. These little pieces are common finds on Roman military sites. I also recall hearing somewhere (any resident chemists to affirm/refute this?) that chemical reactions between the bronze fittings and iron plates fostered corrosion. It also took much more technological skill to produce a plate cuirass than one made of mail.
That said, most modern tests I’ve seen seem to have confirmed the efficiency of segmented armor. It’s plates (unlike those of later medieval plate armor) were not hardened by forging which allowed the softer metal to absorb a blow, spreading its force. Mail can be pierced by pointed weapons and projectiles and shattered by blunt ones, but segmented cuirasses are far harder to pierce and much better suited to evenly distributing the shock of blows (I use the present tense because I am referring to present-day tests, which very well might be using higher quality iron than was available in Antiquity). The lorica segmentata also appears to be the preferred choice of cuirass amongst rein actors from what I have seen. Perhaps someone who knows more about these events can clarify why.
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