[Bahamut]

There's a lot of old cruft using async (my current workplace being one of them) - bluebird also has 4x the downloads of async.

async is pretty terrible though, you cannot pass the results from one execution to another, i.e. passing results from one function in async.series to another, which results in developers tending to pollute variable scope by defining above and filling it in inside each callback. This prevents the ability to write clean isolated testable functions.