I wouldn't even classify CoffeeScript as another language. It's really just javascript with some syntactical sugar. It took me about 20 mins to get comfortable with it.
It's another barrier to entry. Documentation is probably the worst place to encounter a new language, since the reader is already confused and frustrated.
CoffeeScript is a nice language, but I think it's reputation suffers from it's association with Ruby. Far too many CoffeeScript tutorials are aimed at Rails devs who don't want to learn new syntax.
It's a lot of syntactic sugar and things removed. Even the ?: operator which I use a lot. Typing "a = if b then c else d" is a huge wasted opportunity to just keep the ?: op. Plus there's no way to get the == operator even if you know you want it and understand the nuances.
I honestly find I use the ? operator in coffeescript far, far more often than I want to use a ternary conditional.
Fair enough on the == operator, but it's another that I have honestly never needed or wanted.
It's just one extra barrier of entry for non-coffeescript developers. I personally thought about writing a simple plugin, went to the documentation and realized it was all in coffeescript with no JS option, and gave up. I simply don't have the mental capacity to learn another language that probably won't be around in 10 years.