Both of those lack support for custom "sources", instead of just markdown files.
A lot of people like Contentful (https://www.contentful.com/), which static site generators like Gatsby support.
This doesn't really address your remark about webpack though.
However, I have been using Gatsby and I haven't had any issues with it. It is completely internal and hidden from the end user.
For me that's the beauty of it, just simple structured text files. If custom sources are needed for a static website then writing a transform script to generate the markdown files and yaml header is simpler than having to deal with integration apis.
That's why I wrote sitio[1]. sitio has a pretty simple imperative API where you can say stuff like "generate page on /about/ using the React component on 'components/page.js' and the following data: {}", so your entire configuration is just a simple nodejs script and you can use data from anywhere.
Then it generates static pages that, if JavaScript is avalaible, will become React components and the entire site will be a Spa with proper history. If there's no JS, it's an HTML static site.
Also, it uses Browserify under the hood, because Webpack is horrible.
Does it generate pure html, without react support?
Middleman / Jekyll both support slim, webpack if needed, live reload, json or yaml data files, partials, page matter (in-page data in yaml format) etc.
The only issue is setting up webpack (for vie and Babel in the middleman specific confit).
Also, Hugo lacks support for plugins. If you want to do something with it, it better be baked-in out-of-the-box.