Also: avoidance of mutable state, even locally. A lot of functions can be more straightforwardly expressed using mutable variables and data structures, and most functional languages can handle local mutable state, but a lot of code in functional languages avoids it.
Conversely, a lot of code written in imperative languages would be clearer and/or less bug-prone if it avoided mutable state and used persistent data structures.
I wish there was a mainstream, high performance language that made both styles equally ergonomic.
In my experience the main benefit of functional programming is function purity. I am completely fine with mutation inside of a function since the all of the mutation logic is self-contained in a single small block of text.
I think everyone should take a shot at writing a non-trivial functional program to see the benefit. Once you understand what makes it great, you can apply what you've learned to the majority of OOP/impure languages.
Rust is very good at making things immutable. But that doesn't mean it's particularly fun to, for example, append or prepend something to a list and retain a reference to both the old and the new list.
Compare to most "functional" languages, in which prepending an item to a list and ending up with immutable references to the old and new lists is almost the defining feature of the language.
Considering the performance implications of a full commitment to immutable data structures, I think Rust does a pretty good job here.
Dynamic arrays / vectors / slices or whatever you want to call them are probably the most important or fundamental non-plain-old-data data structure, and in a purely immutable environment these are essentially impossible (or have awful performance characteristics).
Yeah although I find FP languages' obsession with singly linked lists to be a real flaw because they have such awful performance characteristics. And also pretty bad ergonomics often. The number of times stuff comes out backwards...
Flix has "regions", which are a way to allow mutation locally while keeping the language purely functional otherwise: See Region-based Local Mutation in https://flix.dev/
Conversely, a lot of code written in imperative languages would be clearer and/or less bug-prone if it avoided mutable state and used persistent data structures.
I wish there was a mainstream, high performance language that made both styles equally ergonomic.