I found this to be true when building a SPA. On rails I am already told the best place to put things but on the JS side its just "do whatever you want" so I try something and find out later that it wasn't a great way to do things so I am just wasting my time working out the best way to organize a project. I also have to deal with index files that just import every single component for the frontend when on rails this kind of stuff is auto generated.

Rack is cool, but the modern approach seems to be more decoupled web servers. You rarely see multiple business-logic Rack servers in a single process anyway though; Rack servers outside your primary business logic tend to be used more like Express middlewares in Node (which are very conventional at this point).

And the standard entry point for a NodeJS package is "whatever is defined as the main property in your package.json." Do otherwise at your peril.