I found it fairly simple to build Gradle plugins with Kotlin. If anything, the problem was just having the patience to actually find the right documentation in the first place, and understand what was being described. The main problem I faced there was that I wanted a plugin to configure dependencies for the project it would run against and the docs around dealing with dependencies and detached configurations were a bit confusing.

I do find it curious that a lot of these tools get seen as basic task runners despite offering much more potential.