From what I remember, Polymer before 1.0 had some severe performance issues on browsers that didn't have native Web Components support, because it shipped with

From what I remember, Polymer before 1.0 had some severe performance issues on browsers that didn't have native Web Components support, because it shipped with a polyfill that tried to emulate the shadow DOM properly. I suspect (but have no evidence) that this caused lots of people to dismiss it because of that, and they might not have taken another look at it since 1.0 released.