Was samurai armor effective?

From the Nara period (710-784) to the Genpei War (1180-1185) there were some major innovations in Japanese military organization, one above all the massive introduction of cavalry. This renewal entailed several changes in the structure of armor, which until the 12th century had a plate construction not unlike that of other peoples: horsemen had to wear very heavy armor, so as to ensure stability during charges and protection from enemy weapons. These ō-yoroi (literally "great armor"), were characterized by a dense weave of small iron plates tied to each other, and are the model for what will remain the typical Japanese armor as we know it today. Most warriors were equipped with bows and arrows, and consequently armor was designed primarily to repel these kinds of attacks. Therefore, it would be a mistake to judge the effectiveness of this equipment in relation to different types of combat! In fact, at this stage of Japanese history the military arts were still underdeveloped: there were no large armies yet, and disputes were mostly settled by brief skirmishes. We also have evidence that, in several cases, the outcomes of battles were decided by a duel between the commanders of the different sides. These warriors, often seen as epic heroes, could count on armor of extraordinary workmanship; the battlefield thus became a time to show off one's equipment, which also reflected the prestige of the warrior and his family. Other types of armor were also commissioned during the same period, with a special purpose: it was customary to offer them as gifts to Shinto shrines and thus not intended for use on the battlefield. Among the most important of these votive armors are those preserved at the Kasuga shrine in Nara, perhaps among the greatest emblems of the craftsmanship of the Kamakura period.
When, then, did these samurai armors "lose effectiveness"?
In the early 13th century, during the Kamakura period (1185-1333), something unthinkable happened in China: the Mongols, led by the formidable Genghis Khan, were able to invade and conquer much of the Chinese empire. A few decades later Qubilai Khan, Genghis Khan's grandson, founded a new imperial dynasty with the aim of dominating the entire Far East. One of the Mongol emperor's first targets was Japan itself, a prosperous independent nation with no experience in warfare. 
In 1274 the Mongols assembled a fleet capable of carrying an army of 15,000 men: departing from Korea they quickly reached Kyushu, the southernmost island of the Japanese archipelago. The armies there were completely taken aback by their opponents' military tactics: the Mongols in fact fought in well-disciplined battalions, organized by phalanxes and equipped with both long-range weapons and swords and spears. Cavalry, on the other hand, the Mongols' secret weapon during European campaigns, was absent, since transporting horses by sea would have been difficult and costly. The invaders were also equipped with leather armor, light and comfortable, ideal for the rapid maneuvers that distinguished the Mongols' way of warfare. In the face of this deployment of forces, the "ritual" customs of the Japanese warriors soon proved inefficient if not entirely suicidal, as both cavalry charges and individual dueling proposals resulted in ruinous defeats. 
The warrior class emerged seriously diminished from this first confrontation with a major foreign power. Tactics perfected for years had proved utterly useless in the face of the might of a regular army. The samurai's equipment was also expensive and too heavy: the ō-yoroi were designed so that they could repel the violent impacts of a cavalryman's charge, but they were useless in foot combat, which required agility of movement. Japanese armorers therefore set to work on improving the samurai's equipment, focusing mainly on reducing weight and perfecting protective plates. In time two new types of armor were created, aesthetically not very different from their predecessors, but with some innovations that made them immediately popular. The two new models, called haramaki and dō-maru, were markedly lighter than the ō-yoroi, and, thanks to improvements in platelet production and lacquering techniques, the armor became even more solid and reliable.

However, let us now analyze another aspect related to effectiveness: not the effectiveness of individual armor but that of an entire army: how long does it take to build a large number of suits of armor? How quickly are these armors repairable?

By the first half of the 15th century, the shogunate was now considered a marginal political body, similar to the imperial court. Because of the lust for power of the daimyō, the large landowners who had full military and political control within their fiefdom, a period of great internal strife opened up for Japan: the Sengoku jidai (literally "period of fighting states"). During this period, characterized by a state of continuous warfare, Japanese military arts could only be further refined. The armors of the Sengoku period are quite different from those of previous centuries: the need to make movement agile for fighters and the need to produce efficient and easily repairable equipment in a short time led to numerous revisions of traditional techniques. Thus the weight of armor was decreased, shoulder straps shrunk, and protections on helmets (shikoro and fukigaeshi) reduced or eliminated. Silk bindings, prone to breakage and definitely inconvenient on rainy days when they became soaked and unnecessarily weighed down the samurai, were reduced to a minimum. The upper edge of the breastplate was then folded outward, so that blows received with the pole arms would not slip out of the guards and hit the throat; new helmet designs were also created, suitable for the new fighting styles but at the same time quick to make. All these elements came together in a new generation of armor, known as tosei gusoku ("modern armor”).