I sometimes get this. I've always found it to be a caching issue (particularly on Windows). Try:
npm cache clear and re-do npm install
This is pretty cool but I worry that it's another complexity/wrapper onto of gulp to worry about for little gain -- Gulp and grunt files really aren't that difficult to setup, they are so minimal that the argument that it's "easier" isn't really that valid. Also the linear nature of having one build file where you can see all your build steps, exact paths and configuration in one file is incredibly valuable, particularly when your working on a project you don't know much about. The only thing that I can completely see the benefit in having is the event/route scanner, or anything that is Laravel-specific, but to me that should have just been a gulp/grunt plugin that you can use in whatever build tool you like.
Though I'm willing to give it a go (even though I'm not a fan of Gulp but meh). One thing that makes Elixir unusable (for me at the moment) is that during development it would be incredibly slow to use. I don't like my build process to take any longer than 500ms when constantly updating JS, Less files etc. But it will with Exliar because by default it seems to be minifying and uglyfying your css/js, which massively increases the build time. It would be good to have a dev/debug.
Also it would be good to document how to set custom options to pass e.g. to your less task (if that's even possible?)