I think that Tom Dale made a pretty good argument of why to avoid CoffeeScript as part of his argument for TypeScript.
"For example, CoffeeScript adopted for..of loops to iterate over the properties of an object. Meanwhile, ES6 introduced its own for..of loops as a way to loop through iterable objects. If CoffeeScript wants to support new functionality with similar syntax like this, it has two choices: break existing CoffeeScript code, or diverge from JavaScript more and more over time."
"For example, CoffeeScript adopted for..of loops to iterate over the properties of an object. Meanwhile, ES6 introduced its own for..of loops as a way to loop through iterable objects. If CoffeeScript wants to support new functionality with similar syntax like this, it has two choices: break existing CoffeeScript code, or diverge from JavaScript more and more over time."
https://medium.com/@tomdale/glimmer-js-whats-the-deal-with-t...