> The event-driven concurrency model makes it easier to write servers without worrying about race conditions and thread locks
Well, no, you're just writing the locks yourself in an ad-hoc way. Every time you have a callback calling another callback, you have a lock and all the race condition/deadlock issues associated with that. Of course, writing an application that doesn't have complicated synchronization requirements (streaming fileserver) can often require less boilerplate in an evented system. However, you run into a catch-22 here: by definition it's an application with fewer synchronization requirements, so you'd have to use fewer complicated locks in a 'heavy thread' implementation as well :).
Ultimately it's an engineering tradeoff problem, and you have to weigh lightweight node-style cooperative multitasking with the ability of a traditional thread system to better handle highly complicated scenarios.
Or you can be Russ Cox and argue that this is a false dichotomy and that we should all be using CSP. I'm in that camp.