I ran the numbers, and with a full grid of assault SR weapons (+130% attack), 2x +40% elemental attack is better than 2x +90% character attack (from Lilims) even in best-case scenarios for the latter (3x kamihime of the matching element). In addition, losing the Lilim buff due to a character dying is a significant risk that will tend to cripple the team in difficult content. Thus Lilims are recommended only for newer players who don't have much assault skill or don't have access to better eidolons.