For my part, I found his methodological work very insightful but did not have the time, or perhaps the patience, to follow him in his extremely detailed reconstructions of possible “estimates” of measures of economic activity, and possibly of well-being, essential to ensure the high quality historical studies that he considered necessary, and worth being involved with.
However, I remember his contributions in the 1970s rather well, having read them both at Penn and in Rome, attending some of his seminars and discussions on railroads and real value added, slavery and causality. And he engaged, alla pari, with giants of his field such as Robert Fogel and Douglass North.