According to man rg, any paths as the last arguments to the command are paths to search from. However, using the universal argument C-u counsel-rg to prompt counsel-rg for extra arguments and specifying a path as an argument will result in counsel-rg not showing results from this path and from the initial/default path, so the search results will always have nothing.
Example: C-u counsel-rg, enter ~/bin as initial directory to search for, enter ~/.emacs.d as an argument when prompted, and any string you search will return 0 results even if that string is in ~/bin.
Saw this issue posted on /r/emacs, so might as well submit a GH issue.