For batches of transactions where each transaction has a service call that depends on the response from a previous service call, async especially outperforms.
Rate limiting is not hard either way, but single-threaded async is slightly easier because your counter is automatically thread safe.
The larger and more complex your system gets the harder it gets to keep it thread-safe. For me this is the big advantage of single-threaded async, and I like node because this single-threaded async is idiomatic in JavaScript.
