This is actually not true. If you're using polymer with the full webcomponents.js polyfill, there's a known bug which breaks contenteditable execCommand calls (which means a lot of rich text editors break when you include the polyfill) [1]. The big heavy polyfill is also pretty slow on some browsers.
If you use the newer, lighter-weight shady-dom version of the polyfill, which is a lot more responsive on non-Chrome browsers and doesn't introduce the bug indicated in [1], you must do all DOM manipulation via polymer's local DOM api, in order to properly distribute dom elements.[2]
That means, if you want to use polymer, you cannot use angular or ember (for example). Specifically, DOM manipulation via those libraries ends up just using regular calls against element.appendChild, but those appended elements, if they are polymer elements, end up not properly working (event listeners don't attach and styling modifiers don't apply), because you didn't use polymer.dom to go via polymer's local dom API.
I found out about all of this the hard way, and now I have a webapp that works great on chrome, but users cannot use the rich text editor on anything except chrome. Which eliminates all iOS browsers entirely.
You don't have to do anything special.
1. import polymer html file 2. use it
thats all there is to it.