You'd have to make them weaker against any champion that does not have shields, leaving them weak normally and balanced against shields, and even then it's bad (Sion really likes you being unable to break his shield, and Morg loves more CC immunity you won't get through). Overall, an ability specifically against shields (or passive, or active, or whatever) is going to be bad (either the champion is balanced normally and overbearing against shields, or weak normally and balanced against shields). Even then, it's not something you appreciate much (you lost because your kit has a lot of power in "ignore shield" instead of damage? You lost because the enemy doesn't have or rely on shields?).