Zynthian images does not use master branches for the 2 more important/complex repositories:
The update system uses different branches for updating these 2 repositories. In that way, i can merge from the master only those features that are "more or less" well tested. So developers and advanced testers can use the master branch in its zynthian boxes for testing before merging the changes into the "update" branch and going to "the masses".
Anyway, currently Zynthian software is very experimental and using it, you "kind of agree" contributing to debug. We are a small community yet, and we need testers to reach stability. If only developers test the changes, it wont work, so ... currently every user is a tester
If you want stability you should use the SD image without updating. The updates from a SD image to the next one can contain bugs ... well, the SD images too
Probably we can define a better way of working, and we will do someday, but currently it's not a priority. We are only a few developers yet. Let's walk a little bit more, focus on improving software, adding features and debugging. We will come back to this subject later, when it's really important and needed.