Configuring webpack, mostly. :-D
That’s not a dig at webpack: Those tools are super complex, and hiding complexity from the user is not easy. But it seems that with Vite we finally got there.
I think people just don't want to bother. They don't want to read the docs, or maybe watch a video or two (back when webpack was popular, Sean Larkin, webpack evangelist, made a number of popular courses about setting it up). Also, webpack config became easier compared to 2014/2015; I think they got to practically a zero-config by default.
I can understand that people don't want to care; but "impossible to reason about" is not it. It isn't rust, for crying out loud; nor lisp; nor haskell.