Intuitively you can derive that this is true informally past just an overall feeling. It’s simple really. Functional programming is just imperative programming

Intuitively you can derive that this is true informally past just an overall feeling. It’s simple really. Functional programming is just imperative programming without one feature: mutability. Thus if functional programming is just regular programming with a reduced feature set it means it has the same error surface area as regular programming minus the surface area of errors caused by mutability. Hence by proof the error surface area is smaller. Now think of of all the errors caused by initializing a variable as null and changing it later rather then immediately initializing an immutable variable with the correct value and you can intuit just how big the error surface area actually is.