I have had units myself with no unit models. These are not selectable with mouse-click-drag but selectable in right side list of units. Restarting game did not bring models back. This happened after my successor changed both religion and culture group through education (from arabic to persian, sunni to ibadi). This example is of a camel retinue unit but I have seen raised levies with similar problem (after 2.2 patch). It's not game breaking but since click-drag selection doesn't work on them, it's certainly annoying to select and move several.
