Molecular dynamics (MD) simulations of solids flows provide a useful tool for extracting transport coefficients needed for the closure of continuum models This "empirical" approach is particularly powerful for systems with complex interactions-nonspherical particles, cohesive particles, etc.-that are difficult to handle with first-principles (e g, kinetic-theory-based) approaches Nonetheless, the extraction of such quantities is characterized by several subtleties which may lead to inaccuracies if ignored. In this work, four such complexities are illustrated (i) the presence of clustering instabilities, (ii) the a prior: need for the general form of the flux law, (iii) the presence of more than one transport coefficient in a given flux law, and (iv) the presence of Knudsen (higher-order) effects