Gradle. I appreciate that it is a fast build system, and a lot of it does just work. When it doesn't just work it's a nightmare. The config language is completely opaque and undiscoverable (Kotlin might fix this, but I ran out of patience to understand how Gradle works a while ago) though.

In many respects I think the fact there's a commercial version of it is a sign that it's lacking in the UX area.