I think right now there isn't much reason to use Bullet Punch over Knuckleboom. As it stands, the 3 hit combo is much more practical while the charge of the alt is incredibly slow and doesn't really feel like the damage you get out of it is even that great over the practicality of the normal skill. It might be the most cumbersome primary attack in the game arguably.
Also with a name like "Bullet Punch", I was very disappointed it doesn't launch loader forward or even pierce enemies much like his counterpart in 2 has. If it at least had this utility I could understand the long wind up period and inability to move much with it, plus it would just be way more fun. 