ES2015 and ES7 are currently state-of-the-art web languages;
Ember and React are extensively using ES2015 and ES7 (!) features in their core features;
if we want to publish interesting content on either blog or Stack Overflow, we need to switch between Coffee and JS, as JS is a default way in modern work;
even though some people say that JS syntax is worse, the reason might be the lack of ES2015 knowledge, which makes simplifications like arrow functions the same as in Coffee;
better tools - we can easily add linters to our projects that are quite smart about JS code; ember has built-in linter config that fails tests if our code isn't proper; CodeClimate supports JS.
some people say that JS syntax is worse than Coffee;
it will bring confusion due to different syntax in different projects (older projects will stay in Coffee);
there is no easy and working transpiler between Coffee to JS (ES2015) and it demands simple transpiling to ES5 and changing by hand (error-prone);
we will have to maintain old coffee projects and new JS projects.
it's our default now, and all of us are familiar with coffee;
it's easier to work with and maintain due to a simple syntax;
staying with coffee won't demand any change and confusion while switching between old and new projects.
it doesn't support ES2015 features;
no support from CodeClimate or JSLint / ESLint, built in Ember as a default tests part;
due to the lack of ES2015 support, we cannot use some of the new features in Emberjs like: new computed property syntax using getters and setters, importing modules ES2015 style and others;
it's hard to work with the newest features and examples from Ember or React due to the fact that they are written in ES2015 and shown by JSBin that supports ES2015;
These pros and cons are highly opinionated and might be different for you, depending on projects you work on. We'd be happy if you could share your perspective in the comments section below.