For writing production code, I find good scoping rules non-negotiable. And error handling, monitoring etc has to be well thought out before deploying at scale.

So as great as Mathematica sounds for interactive math and science computations, sounds like a poor tool for building systems that will be deployed and used by many people.

That is a fair assessment. By and large it is used for the former. It is super handy in the exploratory phase of certain kinds of mathematical research.