[Wintamute]

In a large complex project build scripts tend to take on many tasks, and some of them will not necessarily fit into the pattern of taking in a stream of file globs, modifying them and piping them on into the next task. For example your script might manage a Git repo, deploy an app to Heroku, rsync some files somewhere, run some tests in a headless browser etc. So a build system that's optimised around streams, and one that you have to script by hand, might be very well suited for small streamlined projects but once things get varied and complex it might better be better to define your tasks via configuration instead of code, i.e. Grunt.