[1qaz2wsx3edc]

As someone who used polymer/paper-elements recently in a project, these are some downsides:

1. The whole design guide is not implemented (date picker for example)

2. Web components polyfill while pretty fast still slows things down a bit

3. Extra effort required to debug/style components (shadow DOM)

4. It's still fairly bleeding edge, not for the faint of heart

5. Vulcanizing required for assets to avoid hundreds of imports/requests

These are some of the factors that would make me reconsider using it on another project. It still turned out very nice for the project however.