deserializing data structures is a great example. JSON.parse is SLOW. all it takes is a bit of bad code that doesn't do any checking of input size/etc... and if an authoritative source starts sending larger payloads than usual, your node process is horked. Granted, that's not a problem remotely specific to node, it just has a bigger effect in a node env.
Sure, JavaScript (and Ruby and Python) are usually fast enough with modern hardware - but we can't pretend there's no cost.