How impressive is Stannis’s victory over the wildings, militarily speaking? On the one hand, the fighters outnumbered him 20 to 1, but they had poor unit cohesion and he was vastly technologically superior.

Vastly is a strong word. Compare his technological superiority to the wildlings to that of the British (who still got their asses kicked regardless) vs. the Zulu at Islandlwana, it’s nothing. He’s got more horses and more armor and more steel, but that’s less of a radical change than you think. Also, they have mammoths and giants. 

As for poor unit cohesion – well, the flip side of that is that Stannis had very good unit cohesion and achieved the element of surprise preventing the wildlings from forming shield walls.