They don&#x27;t re-evaluate on nested path changes, e.g. if (item.bar.isActive) won&#x27;t trigger if isActive changes. You can work around this in practice but often it requires significant architectural changes and for your regular JS code to be Polymer-aware.<p>This wasn&#x27;t true in Polymer 0.6 but required Object.observe which is SLOW.<p>React&#x27;s render everything and diff approach avoids this issue.