Extremely difficult in maven and five minutes work in make or ant is things like "running LaTeX to generate a PDF of some of your documentation, because it is math laden and HTML isn't suitable". This is not theoretical; I had to do this at the place that was using maven and we depended on some semi-obscure mathematical formalisms like the stable distribution.
Intermediate between these two is driving things with maven plugins, which is either easy (because you're doing something trivial) or fiendishly difficult because the debugging output is terrible, the docs are worse and the only thing left to you is to Use The Source, Luke. Every nontrivial maven plugin I've ever used has tripped this at some point, be it loading config files, including the dependencies, or something else.
I agree that not every maven plugin does exactly what you want it to, but a lot of them get pretty damn close, and as the authors iterate them, they gain a lot of flexibility.
