Duby was writing before the turn in historiography that rejected feudalism as a useful construct. Thus much of his work that dealt w/ the structure of medieval society, The Three Orders, The Chivalrous Society, and The Knight, The Lady, and The Priest is considered to be outdated. He also had a somewhat looser relationship to citations and a slightly lower threshold for his standards of evidence than we have now. Sometimes his work can feel a bit naive. In William Marshal, The Flower of Chivalry, for example, he declares that Marshal must have done many heroic deeds on the Crusade...even though we have no sources that discuss this. All of that said, Duby was an extremely important historian, and his work set the stage for the current state of the field of medieval studies. Consequently, Duby features often in exam reading lists, and we still discuss his work. He also was an incredibly fluent, beautiful writer - a poet, as somebody once said - and thus his books are still very enjoyable to read.