[Risord]

Don't have experience about stack oriented languages nor sure is this relevant reply.

But to clarify: local mutable variables are fine because effect of changing value is clear due well specified scope. Mutation is not observable from outside of a function.

Opposite is true for using mutable data structures (with immutable variables): scope of effect must be checked manually, it's easy to leak function abstraction and with 3rd party code all the bets are off.