Generally speaking, you want to be right in people’s faces (well actually more mid-ish range with fragger, but you get the point) and keeping pressure on with a steady stream of bullets. The K-121 is way more powerful by design, its what Fragger does. He kills stuff. Thats his role as an assault.
So its not so much that the burst rifles are bad, they just aren’t what you want on Fragger most of the time, although a few people swear by them.