Ractive.js vs Polymer

Ractive fully supports a templating language. To be more precise, views are written with a variant of Mustache, which is also extended to support inline JavaScript expressions. Soon it will be able to support other templating languages.

Ractive has a concept of proxy events, which translate a user action (e.g. a mouseclick) defined via an event directive into an intention (e.g. 'select this option'). This allows you to handle user interaction in a readable, declarative fashion.
on-click='activate'
with arguments:
on-click = 'activate: {{a}}, {{b}}'
It's activate (and not click, nor your function name) that is the name of the handler event that will be fired for any registered handlers created via
ractive.on('activate', your_handler)
ractive.on('activate', your_another_handler)

Of course, Ractive also supports method calls like on-click='toggle(foo)'

Two-way binding can be turned off by those that are concerned it may be a source of bugs.

They have a great interactive tutorial which makes the learning process easy peasy. You will get into it within a couple of minutes.

Instead of relying on the DOM, Ractive implements a virtual DOM from scratch, allowing it to calculate precisely what needs to be patched during the next screen refresh. This is orders of magnitude faster than fiddling with the DOM itself.

It provides a base component.

HTML markup as it can be a non-string.

It provides Flex layout components.

CSS can be applied far more comfortably than React.

The presence od specialized debugging tools are advertised by competitors. The all features of web components are natively supported by browser embedded development tools.

The router is embedded into CLI for project creation and covers as web as Progressive web app, also fused with Polymer layouts out of the box. The shop template for CLI has a complete solution including the routing.

Full app stack from data tier to routing, progressive web app, responsive layouts makes no need to seek outside of Polymer ecosystem for application features.

In addition to waste set of mature web components in Polymer Elements along with Vaadin Elements there are thousands of web components in the wild comparable to jQuery plugins set.

Polymer guides you as with tools (cli, build environment, app templates,..) as with complimentary documentation on all phases of app development from creation of app as progresive web app to production deployment instructions.

As Polymer is standards based, the whole community around those standards also helping in documentation and support.

Web Components are a collection of specifications released by W3C as a way to reduce the complexity of web apps by creating reusable components. Browser support is currently poor for web components, however Polymer is developed to make web components compatible with modern browsers.

The Polymer APIs are split on application layers and follow standards on all possible ways: Web Components, CSS variables, async API via Promises and so on.