Not just more difficult. Threads are really wasteful to spawn just so you can hit 10 shards. Even if you have a thread pool. Evented I/O is much better than threads.

When NGiNX launched, it was the evented web server in a sea of threaded servers. Guess which paradigm won.

It's not a dichotomy. The answer is that both models are useful, and most systems support hybrid threaded and evented modes. Node.js only supports one model, which is a huge drawback.

threads don't have to be more difficult. check out actors.

Meteor seems to think threads are the way to go (Fibers)