I am noticing some notable package maintainers are choosing to not use the Emacs package management system (ESS?) or complaining about its limitations (Helm).
WARNING: Due to a bad concept of package.el which is in charge of fetching helm files and compiling them, users had errors most of the time when upgrading from melpa and list-package. To avoid this Async have been added as dependency to helm to force package.el compiling its files in a clean environment. People installing from git and using the make file will not suffer from this problem and don't need Async though it is recommended as it fix installation of all other packages you may install with package.el from (m)elpa. See FAQ for more infos.
What exact technical limitations does the current package management system has that they could be alluding to, and why would packages need to use
async as a dependency?