What was the functionality?

If you stick to defining dependencies maven works perfectly. If you encapsulate whatever other crap you want to do as proper maven plugins maven works perfectly. It's the people who decide they absolutely need to output to a different folder on Tuesdays but can't be bothered to write a proper tested plugin for doing that who get themselves in trouble.

It was an Atlassian plugin, written in Scala. I also wanted to download the JS Closure Library and minify JS with the Closure Compiler.

Your comment is accurate. If you want to do a very specific set of things, Maven works fine. If you want anything outside that, it gets much harder.