On the other side I'm using Java with Maven and it is - a big burden. It's build in dependency retrieval system also isn't helping. Maybe I just don't like XML - because it is not human-readable ;)
The other big one is that you need to have a strong familiarity with Make, and use it often. Familiar because there's no way a person who hasn't actually been through some form of Make documentation in detail can even guess at what things like $@, $?, and $^ do, or accurately decipher its macro replacement syntax, or any of that. And use it often because, even if you were deeply familiar with Make in the past, if you haven't touched it in a few years, you're unlikely to reliably remember it without help.
If you use it Maven-style, with relatively simple build scripts and all the complexity pushed out to plugins, it's very readable. A fair bit more readable than Maven, in my opinion, but I wouldn't care to argue that point in particular.
If you use it the way Stack Overflow tells you to do it, so that your build scripts are basically glorified ad-hoc Groovy programs, then, yeah, it's just impossible.
Unfortunately, since the only way to really understand any of Gradle is to understand all of Gradle, and understanding all of Gradle is a huge effort, it's kind of a catch-22. For most people, the only really sensible way to use it is to Google for advice and then do what Stack Overflow tells you to do. But that invariably leads to a result that is not even remotely sensible.
Long story short, my hot take is this. Maven is write-only. Gradle (ideal usage) is read-only. Gradle (normal usage) can be neither read nor written.
:) it devolves into a custom shell script