Pelican is way more customisable, you can write Python scripts to do whatever you want. The downsides: you need to have a virtualenv and that it is pretty slow (especially syntax highlighting). It is awesome overall though, my favourite after Zola ;)
I'm currently using Zola + python/js scripts to do things Zola can't and considering a move to something like Pelican.
In practice to use PostCSS, you need npm to install plugins, and maybe a `postcss.config.js`, so there are no advantages of having that PostCSS call in Zola's `config.toml` or as a script in `package.json`.
I don't really see how others SSG would handle that. Hugo just shells out a command to `postcss-cli` that needs to be installed (https://gohugo.io/hugo-pipes/postcss/), along with whatever plugins you are using. It's not really something "built-in".