Web Components are a collection of specifications released by W3C as a way to reduce the complexity of web apps by creating reusable components. Browser support is currently poor for web components, however Polymer is developed to make web components compatible with modern browsers.
Due to web components being so new, there is still a lot of issues with browser compatibility. Though Polymer has solved some of these compatibility issues with polyfills, there is still a lot of bugs and unpredictability.
It's really difficult to build a snappy interface with Material UI. Maybe this is React, maybe it's just the way it's built. A relatively simple form driven from a list with > 50 elements was slower than it should be.