Postinstall scripts are the main issue, yep. Right now the current implementation doesn't do anything special with them, meaning that they are installed inside the cache (except when they're disabled altogether, which often works well enough since there isn't that many packages that require postinstall scripts).

This is obviously wrong, so we'll soon go to a model where we "unplug" the packages and put them into a specific directory (a bit like the node_modules folder, but entirely flat). The feature itself is already there (`yarn unplug` in the rfc), but we need to wire it to the install process.

Ideally, I think it would be beneficial for the ecosystem to move away as much as possible from postinstall scripts - WebAssembly became a very good solution for native libraries, and as the added benefit that it works everywhere, including the web.