Having worked with Maven and Gradle, I'd say Gradle was worse in the average case, but better in the worst case. There are way more Gradle projects with unnecessary custom build code because Gradle makes it easy to do.
On the other hand, when builds are specified in a limited-power build config language, like POM, then when someone needs to do something custom, they have to extend or modify the build tool itself, which in my experience causes way more pain than custom code in a build file. Custom logic in Maven means building and publishing an extension; it can't be local to the project. You may encounter projects that depend on extensions from long-lost open source projects, or long-lost internal projects. On one occasion, I was lucky to find a source jar for the extension in the Maven repository. It can be a nightmare.
The same could happen with Gradle, since a build can depend on arbitrary libraries, but I never saw it in the wild. People depended on major open-source extensions and added their own custom code inside the build.
When I used Maven, extensions had to be published to and pulled from a public repo. We couldn't even use the private repo that we used for the rest of our libraries, because the extension had to be loaded before Maven read the file where our private repo was configured.
Whereas a Gradle build can read Groovy files straight from disk.
Then you don't have a standard build, you have a build with multiple steps that needs to be documented and/or scripted. In an organization where every other project builds in a single step with "mvn package", and people can check out a repo and fire up their IDE and stuff just works, people are going to get bent out of shape because from their perspective, things aren't working out of the box.
A slightly more powerful build tool that supports custom code in the build doesn't force users to script around it. You can create an arbitrarily customized build that builds with the same commands as a Hello World project. (It's a double-edged sword, to be sure, because people don't try as hard to avoid customization as they would with Maven.)
You can, but why should you need to? Why can't the build tool take the plugin code directly off of disk, build it, and use it? This kind of orchestration of manual steps is what build tools are meant to be good at
Sure. But adding ability to self-modify the build drastically increases the complexity of a build tool. Maven developers decided that they want to avoid that.
I'm using several maven plugins (not extensions) that are defined within the reactor project itself. It works well.
You do need to split your build into multiple projects governed by a reactor but you'll have that anyway as soon as you have more than 1 module. Then you just always build the reactor. Pretty much the same idea as gradle.
1: I believe that you encountered errors, programming is packed to the gills with them, but correlation is not causation in that just because it did not immediately work in your setup does not mean it's impossible or forbidden
On the other hand, when builds are specified in a limited-power build config language, like POM, then when someone needs to do something custom, they have to extend or modify the build tool itself, which in my experience causes way more pain than custom code in a build file. Custom logic in Maven means building and publishing an extension; it can't be local to the project. You may encounter projects that depend on extensions from long-lost open source projects, or long-lost internal projects. On one occasion, I was lucky to find a source jar for the extension in the Maven repository. It can be a nightmare.
The same could happen with Gradle, since a build can depend on arbitrary libraries, but I never saw it in the wild. People depended on major open-source extensions and added their own custom code inside the build.