All of these are valid points except 1. Node's async IO is one of its strengths. Contrast with Rails, for example, where the standard practice for concurrency is to spawn multiple processes (or, less commonly, threads). How many Rails processes can you fit on one machine? 5-10? Node can handle thousands of concurrent connections, all on a single thread. And when you hit those limits, you can always continue scaling with multiple processes like you would with Rails.

Doing CPU-bound computation in your application server is an anti-pattern. IO-bound computation, however, is where Node excels.

The thing to realize is that pre-emptive multitasking is costly. It is convenient for the programmer (the programmer doesn't need to worry about blocking and locking up the rest of the program), but it comes at a cost. Lightweight "green" threads, or equivalently, Node's evented dispatch mechanism, are a much more efficient use of the CPU. For applications composed of short-lived computations (e.g., < 1ms), it doesn't make sense to interrupt them and context switch. It's more performant to let them complete and then switch. You just have to make sure you aren't doing any CPU-intensive computations in the app server—which you probably shouldn't be doing anyway.

The downside of Node's approach is callback hell. And that's why we have Go.