A couple of down sides of web components: - they interact poorly with requirejs and amd in general. - they flat out dont work at all in IE8 - there's no 'good' solution for the request hell you get from loading 50 components; although some of the tooling is getting there (vulcanize). - the tooling in general is currently rubbish; no way to easily combine sass + jade + coffee into a web component (eg. with gulp, no th…
With ReactJS you get to code as if everything is a component, why use polymer now instead of that?
@jastanton phrased it nicely in another comment here: https://news.ycombinator.com/item?id=7971847