What measurements do you base the scalability conclusion / comparison of python, ruby (rails), and nodejs on?
nodejs maintains concurrency by events, it's common sense that a single thread would never be able to equivalent of multi-thread.
Now, let's talk about Ruby vs Python.
Python requires the developer to be clean about side effects and isolation. With Ruby one can write concurrent programs that operate on multiple cores easily, similar to Python, a developer is responsible for side effects and isolation issues. Python’s concurrency process is more resource-demanding as compared to Ruby. But then again, it boils down to developer coding habits if one has to take the cake offered by both Python and Ruby Performance languages.
Seems like a banking system would have its scalability depend more on the transactional database in use, rather than the choice of language used for the api layer, which probably just translates from/to http+json or whatever transport and message standards used, to some atomic sql / db query in a transaction and serialising the response. Seem unlikely there would be any need for shared memory multi-threading in the api layer. Node.js could probably be used just fine, e.g. with PostGraphile on top of PostgreSQL. It seems very unlikely to be the bottleneck.
In case of online banking, bottleneck may occur. I'd rather prefer isolated hybrid database API, that'll better than PostGraphile. For security issues, I'll not recommend any GraphQL API in online banking application, also it'll not be helpful in this scenario. It's true that "a banking system would have its scalability depend more on the transactional database in use", but when it's about online banking it's not only about transaction, I bet, they've more to offer to their clients. So, scalability on api layer is also important for this case.
Brother I have been working in Ruby on rails and nodejs from last 3 years. From my experience it is easy to scale nodejs as compared to Ruby and it even requires less resources for deployment as well. This was one of the reason why linkedin shifted their stack from Ruby on Rails to Nodejs.
