Wouldn't map and reduce be more readable if you were used to reading it? I've been using a lot of Elixir and Ruby, both of which use map and reduce, and neither of which make extensive use of for loops. Map and reduce are what I am used to reading.
When you have some imperative code inside the reduce function and you have to go out of your way to achieve something (ie, immutability by using Object.assign, spread, or Array.prototype.concat, etc) and on top of that have some conditional and imperative code, then the reduce function stops being elegant, and looks horrible.
Sure, every problem can be solved in a functional way, but JavaScript is not a purely functional language. Why limit yourself to a subset of the language? IMO, you should use the best tool you can for the problem. Sometimes that's reduce, sometimes it's not.
Also, I love Ruby (and used Elixir), and it's not really a comparison that makes sense, they're different languages with different APIs, syntax, and there's a category of problems in JS you never have to think about when using Erlang/Elixir, or Ruby. I could say that I'm used to seeing maps and folds in Haskell code; doesn't really mean anything in this discussion.
