Depiction of imperial reforms

[Watchman]

I'll deal with the second part of your point first. Iron helms were never in widespread use during the principate. The fact is that most iron helms are recovered from rubbish tips whilst most bronze ones were lost. Bronze was usually recycled, as evidenced by the satripping of bronze fittings, and silverering, from iron helms. In other words it's an archaeological mirrage.

Bronze was also AFAIK relatively valuable, and unlike iron doesn't suffer overmuch from the "march of time". Iron was and is conversely quite abundant and relatively cheap. Guess which would have had the priority for recycling ?

Anyway, that's beside the point. The point is that they had the technological savvy to make good single-piece iron helmets; that's certainly enough to make the lames in decent metallurgical quality (especially as the manufacture of the LS was tricky and demanding enough to have to be left to the "higher tech" manufacturing centers), unless of course some subcontractor was again trying to pull a fast one.

As to the level of protection it basically has to do with the poor design of the armour. Reduced overall coverage in adition to lames that don't really overlap and outright gaps in places make the armour considerably less effective than you might think.

Unless I'm entirely mistaken the lames were leathered to each other in the virtually universal fashion of laminate armour (the sliding-rivet alternative isn't relevant here AFAIK), which would seem to require at least a slight overlap at the edges. Regardless, even a single layer of smooth worked iron plate is anything but easy to get through; and leaving aside the detail I'm somewhat sceptical of the extent of the gaps, actually aiming an attack so it could exploit them ought to have been difficult indeed in battle.

As for the extent of coverage, it's no worse than what you have with solid breastplates and most other cuirasses and corselets, and seems to have been regarded as largely sufficient in most instances (the practice of adding pendant defenses for the upper legs and hips having never been universal).

It has also now been shown that you can get a dagger up between the plates and into the belly.

You can also bypass full plate by stabbing a dagger through the visor or the armpits, and indeed these were among the more common ways men so equipped were actually killed... after first being incapaciated by other means. Doing it while the other guys is still up and fighting back is rather a lot easier said than done however.

Moreover the claim that the LS was a poor piece of armour runs into a bit of a logical problem. Armour design is a field where evolution is rather literally cutthroat, and ideas that don't work get weeded out damn fast in practice. It seems very difficult to believe a culture as pragmatic as the Romans would have expended so much resources and effort equipping as many of its soldiers as manufacturing capacity allowed with these complicated, expensive and logistically troublesome high-tech corselets if there had not been a valid reason to. They were later abandoned as too costly and too troublesome for the needs of troubled times, certainly; but that is a developement of the strategic and logistical spheres that does not as such have any connection to the actual battlefield performance of the item.