I use Polymer 2.0 and it looks like Polymer 3.0 is going to make some React developers happier. But to be honest I prefer the earlier days of Polymer -- it was easier for designers to collaborate with you on projects, we had support for themes, and a GUI builder application.
The main problem that I see is stability. If you've been working with Polymer in the past you've probably rebuilt your apps 3 times to keep up with the changes in the spec. Chasing a spec, seems to be a waste of time. If you compare the adoption of React vs Polymer you see what stability can do to increase adoption.
Going back further to my Swing and JavaFX days, I think there are some really important lessons that one could take from the JavaFX community and shortcut a lot of the API thrashing issues that we've had to deal with. The whole "separate code from UI specification" issue feels like a throwback to the early 2000s. And the need for a GUI builder to make it easy to design an application is something else that was recognized early on in the evolution of JavaFX. Perhaps they could modify the Material Theming plugin to serialize it out as Polymer components.
With Polymer 3.0 a number of the tooling issues should be sorted out. But I'm not exactly sure if you're going to be able to use the Chrome DevTools to click into a series of nested components and find the one that isn't styled right.