Y does this too isn't really an interesting argument that X doing it is reasonable, I don't think (and my response to your initial question is entirely happy to lump Gradle in with SBT on its "understand-ability" demerits).

Don't get me wrong, my laundry list with SBT is huge - that single point was an example; but I have other points for make, cargo, cabal, stack etc...

My question was what makes SBT uniquely bad when the topic of build tooling comes up that people immediately comment how terrible it is.

For me, it's especially bad with the indirection, and the 3-dimensional cube of settings (I can never remember after all these years which one I want to set, the scope or the configuration or the task...). The complete mystery of how plugins affect the final build configuration.

I even went to a lot of effort to master SBT. I've read through the official docs numerous times, and Josh Suereth's Book https://www.manning.com/books/sbt-in-action, and I still feel like I don't grasp it very well. I'm just not smart enough for SBT.

I think a lot of it is probably just that sbt feels as deep as learning entire programming languages sometimes and that is not something I want in a build tool.