I just played through this, solo, vs. all the other AI set to "Hard".
Against the AI, it was a bit of a cakewalk as the Oda. I built max Noburi on the first turn, then stormed over and crushed Miyoshi, stormed north and crushed Azai with a mass of Ashigaru that just kept building, and continued East with that growing horde. I gradually developed a secondary horde in the south and used that to storm West through various neutrals, who died with minimal casualties incurred and then mess with the western powers who had been busy beating each other senseless. In the meantime, I built a third horde to conquer neutrals in the center and forestall adventures by the mostly turtling Tokugawa in the south. My vassals also did their own conquering in a number of cases, starting with Miyoshi occupying much of the center and north, and thus forestalling any Eastern adventures by the western powers. I won on the 13th turn when my vassal Uesugi eventually made Tokugawa into a vassal, even though I only held three capitals on my own.
It seems like Ashigaru become overwhelmingly powerful even if used just as cannon fodder with enough supporting units and leaders.