Polishing or burnishing soft metal (brass, copper, aluminum, etc.) parts with Corn Cob Grit in either a barrel or vibratory tumbler is a popular process. Selecting the proper grit size for small parts or parts with complex geometry can be a real challenge.
There is no right answer to this problem. Balancing the required surface finish with the effort (time, labor, etc.) to achieve that finish will determine the appropriate process for each situation.