...An English longbowman was far superior to firearms (in terms of range, accuracy, stopping power, and rate of firre) probably until the late 1800's, when cartridges were invented. But everyone used guns anyways. Why? Because it only takes about 10 minutes to teach someone to use a gun, and you can churn out piles of guns in factories. It takes decades for someone to become a master archer. Its more effective to have armies of almost entirely inaccurate guns (they might hit something out of dumb luck) than a few master archers.
In general, modern instruments and materials are significantly superior, but we do not have the time to invest into it, or a cultural knowledge of how.
In the "romantic" medieval era, most people would be using a lot of knives, axes and maces without ever being in the military - killing animals, butchering them, thrashing wheat, chopping wood. There was a real world need for armor and weapons, and it flowed in the culture - how to make armor better, how to make it less pierceable etc. We nowadays only inherited the shadow of ancient war techniques, and as such may well fail to properly create a suitable suit of armor, even with having better materials and machinery.
An example of that would be when the opposite happened - metal cuirasses versus bullets did not work at all once rifles and cannons emerged, simply because there was no cultural backbone to prepare for this kind of a weapon. Nowadays, we can craft spectacular bullet-resistant (or immune) suits that can protect you from high-calibre bullets, shrapnel, fire and wrath of Armok, but a trained psycho with a 8-inch blade will kill that person in less than a second.
Also, Quantity > Quantity in many modern world applications.
What would you like. One master soldier, or one thousand recruits? The odds are against the master soldier. Just look at Amazonian warrior ants - they are ANTS that destroy everything in their path (allegedly, they can get a person from living to skeleton in seconds)
To bring the discussion back to the topic, quantity > quality does not work versus armor. I once had a single iron-armored goblin break into my fortress when I didn't have a military equipped. I sent 30 dwarves to wrestle him. They broke a toe, and all died. Seriously? 30 dwarves versus a goblin? A good half of those were trained in combat (not wrestling though) but had no weapons.
But as someone said elsewhere, could well be the problem with natural attacks.