That F-Droid even requires to do the build is one of the reasons I created Discoverium.
Yup. That's a huge, huge issue - IME especially once Java enters the scene. Developers have all sorts of weird stuff in their global ~/.m2/settings.xml that they set up a decade ago and probably don't even think about... real fun when they hand over the project to someone else.
Edit: or perhaps you mean that isn’t the only way to provide such guarantees, which is the implication I got reading your other replies.