Disclaimer: I led development of gaming.youtube.com. We chose Polymer at the time for non-technical reasons.<p>Some downsides:
Performance is not great.
Lack of coherent data architecture (e.g. Redux).
Difficult to manage state change (as opposed to React) due to a more limited programming model - e.g. template ifs and repeats don&#x27;t know to reevaluate in many cases. This makes it harder to integrate with third party js libraries.
Inferior tooling vs React.<p>Philosophically Polymer is more about the DOM and React is more JS-focused. Some of this is personal preference.