I try boza every time I go to Bulgaria, hoping to acquire the taste, so far unsuccessfully. My wife grew up drinking it (accompanied by banitsa) as a schoolgirl in Sofia.  
I have always read (and been told) it is made with millet/просо. (Of course I never thought to read the ingredients before.) This could be the difference that makes Harmonica taste better to you.
There was, IIRC, a boza craze in Europe a few years ago when it was rumored that regularly drinking boza could make a woman's breasts grow larger.