Biggest strength of Leverkusen is from team working because they don't have star players. Some of their players will become star players but for now, they are not. Together by playing and supporting each other, they are making a strong team and seriously challenging Bayern Munich for Bundesliga title.
After the arrival of Xabi, Leverkusen turned into a serious challenger for Bayern Munich, just by relying on ordinary players but Leverkusen right now have proven that they cannot be underestimated, so far we have known Leverkusen only as a mid-table team but now Leverkusen has turned into a giant team in the Bundesliga, the surprise given by Leverkusen makes Bayern Munich now have to worked very hard to be able to win the Bundesliga trophy and they could even lose this season trophy if they are not able to improve their playing performance, If we compare the composition in terms of player prices for Bayern and Leverkusen, it is certainly very different, but as you said, Leverkusen is actually more optimal without any star players in their squad, the even quality of the players is a strength for Leverkusen at the moment and I think they are more united without the selfishness of the star players.