I'm curious what the experience is of maintaining larger codebases in functional languages.

I ask this because it seems like a lot of "big code" issues stem from poor development practices polluting the codebase over a long period of time, destroying any sort of design that once existed.

Does it help if the language takes a hardline stance against mutable state and side effects? I'm certain you can screw those up, but it's confined to a single place.