I haven't worked at a place that has cared about IE since 2009. Besides old versions of IE Polymer works everywhere. Worrying about being backwards compatible with old browsers is a thing of the past with constant updates to Chrome and Firefox. If you're developing a mobile site you can't use any of the newest things until Safari updates (about once a year, seriously) but you should be otherwise ok.

I work at a site where about 20% of traffic comes from IE8. Many are users on corporate machines with no alternatives. I really appreciate that React runs on older browsers and is decently performant without a huge payload. All this with a well designed API and a nice development experience... I think it's hard to beat!

Anyway the more traffic your site gets, the harder it becomes to ignore certain cohorts.

But Polymer depends, for example, on Shadow DOM, which is currently only supported by Chrome, Opera (Chrome-based) and Android browser - not even by Firefox yet (http://caniuse.com/#search=shadow%20dom). There are pollyfills but they're too slow to treat them as a viable solution.