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AFAIK they're impossible to pre-render on the server, and the CSS tricks mentioned in the docs to hide content before components are registered may cause Google's crawler to ignore them. Haven't looked in depth at the React implementation, but the answer seems to tend towards "no".

This is like asking if html elements are SEO friendly. It depends how they're implemented. Google handles javascript now, if that's the salient detail.

Will the post-JS text have the same weight as pre-JS text? Perhaps due to the extra load time or some undocumented aspect of the algorithm.

Depends on how the components are used.

If the content is part of rendered html and wrapped into webcomponents, crawers have no issues accessing that. If the content is loaded dynamically and injected into open mode shadow dom then js enabled crawlers should be able to crawl those too.

If the content is hidden away inside closed mode shadow dom, and also the content is not part of initial HTML then there is no way for a crawler to access it.