USF AT gun which is considered worst AT gun in game
You mean that really cheap ATG that can become the best ATG in the game with the press of a button and some munitions? And become even better with a great vet 1 ability on top? I doubt anyone considers the M1 to be bad. Most consider it to be one of best actually. With its high ROF it's also the best ATG against light vehicles and low armored mediums like the Ostheer P4 and the Ostwind.
Besides, the M-42s dominate light vehicles which is their intended purpose. They do well against mediums, especially with their high performance increases with vet. There is nothing wrong with their AT performance.