The #F-Droid build system can feel restrictive to devs because we require building from source and only use binary build tools that are confirmed to be built from source. That means allowlisting trusted organizations to do the right thing, then looking for exceptions. This is looser than reputable GNU/Linux distros (Debian, Ubuntu, Fedora, Gentoo, etc), which build only with system packages. linsui wrote why Maven Central cannot be blindly trusted to provide free software
Requiring everything be free software is the standard of the best distros, and what #F-Droid strives for. This goes against what Google is trying to with the Android ecosystem. They are pushing hard to get everything using their proprietary bits so that they can control the ecosystem. #F-Droid is the biggest force pushing back.