(Yes, I realize from a technical, end-user perspective this really doesn't matter.)

The Subprocess macro will go through all outputs of the nested activity, query their semantics and search for tracks with the same semantic.

I think some of the design details are insane (I dislike the binary logs, for example), but those are details, not big issues.

Most users frankly don’t care how software is packaged. They don’t understand the difference between deb / rpm / flatpak / snap. They just want a button that installs Spotify so they can listen to their music.

I don't think this is what really matters at the end, since whatever is the implementation the goal should be to provide a library that people actually like to use.

Running the same code in the browser and on the server in order to avoid code duplication is a very different problem. It is simply a matter of good development practices to avoid code duplication. This however is not limited to isomorphic applications. A utility library such as Lodash is “universal”, but has nothing to do with isomorphism. Sharing code between environments does not give you an isomorphic application. What we’re referring to with Universal JavaScript is simply the fact that it is JavaScript code which is environment agnostic. It can run anywhere. In fact most JavaScript code will run fine on any JavaScript platform.

Having the server render the HTML on first page load is the functional part, the thing that provides for a better user experience. The technical part is where we use the same code in both environments, which no user ever asked for, but makes a developer’s life easier (at least in theory).