It becomes very annoying and tedious to refactor when you need to change visibility. Suddenly, a single line change (e.g. changing private to public) needs an IDE to refactor it and make sure it gets all instances (which is quite ironic given that golang proponents generally shun IDEs). Now depending on how many instances changed, you would need to split up your diff for readability, or clutter your diff with needless changes.

The linked article discusses this in the first point:

* Changing the visibility of a variable requires changing its name, everywhere it's used.

* It also cuts contrary to some very long-standing traditions, like using all caps for global constant names, using title case for class names, and starting with lower case for most everything else.