Tried Polymer in a recent project for a basic social webapp last month. Within 2 days we found ourselves reading github issues about to-be-released customization options - options that we needed. The basic features are present alright, but the customization options aren't enough. Integrating modals with custom scrolling, wanting more than 5 options for spacing in the header - why give yourself a hard time when materialize and foundation are waiting with their arms open?