I wanted to use web components for our current project but, as the OP points out, there's no help for iterating through data or even binding data to a source, which is where most of the useful part of a UI comes from. We went with Angular because the components all include data binding and iteration and data testing attributes.

There are plenty of libraries that let you do declarative react templating for web components. I work on LitElement & lit-html, and there are other project list StencilJS. They both let you use fast and convenient libraries to build the component while retaining the native interop of web components.

Right, there are libraries that let you use Web Components as part of a templating system. But notice what's going on with that: WC are now just an implementation detail. You can just as well swap out the Web Component layer and it makes no difference to what's built above it. (For example, Vue CLI has a mode where it will bind to <my-app> instead of <div id="app">. Cool, but does that mean Vue is a Web Component in a meaningful sense?) That's a sign that WC aren't actually solving real problems. There are a couple of semi-useful pieces but no coherent whole worthy of the name.