You can always make your builds do what you want, but the flipside is you can never understand what someone else is doing with their build. I don't think the build system is the place for turing-complete code. Business logic certainly doesn't belong there. Keep the build simple and standardized, and keep code in code.
I've arrived at the opposite opinion over time ... I think people need to recognise that a build system is code. When we pretend it isn't we ultimately end up contorting the system to make up for the missing flexibility. A lot of it looks declarative, but it is not always the case. Sometimes you want imperative constructs. The build should be recognized as code, maintained as code and use a first class language suited to the job.
I do have a problem with Gradle which is that it is almost entirely magical unless you are a pretty advanced Groovy programmer to understand how it is doing what it is doing. I have never felt more disoriented than when trying to learn how to customise a simple aspect of my build and having people post snippets that work but seem completely disconnected from anything else in the build process.
I think that when you do need code in your build that code should be first-class. Which means it should have test coverage, a release process, follow your coding standards and all the rest of it. The maven plugin model encourages that.
And maybe this is making a virtue of necessity, but I find the overhead of creating a plugin stops people from putting random "different compiler arguments on a Wednesday" conditionals in every build, which is all too common if you give them immediate access to a turing-complete language.
> Your criticism applies equally to non-build code. Don't approve PRs for bad/inscrutable code.
But code is where logic goes. People expect logic there. It's the same objection as to logic/conditionals/looping in web templates, or routing config files, or persistence object mappers. If it's logic, it belongs in code!