[zerodensity]

If you care very much about not ever ending up in a bad state (which it seems that you do). Then just functional programming with immutable state is not going to cut it. In this realm you should look into methods of formal verification to prove that your code does what you want it to do.

That being said, functional verification is a huge time sink and is really only economical if it's very important for the code to always be correct. That is, people die if your software has a bug. A few examples would be pacemakers, aircraft autopilots or your car brakes.

If your not working one of these domains then maybe immutable and pure functional code can save you a few bugs? I have not read any peer reviewed study that shows this is the case. But if you have any such paper i would be happy to read it.