why
It's okay with vet. The problem is keeping the vet as it is wiped easily. As someone else pointed out, it's good for P4's and below. The other problem that it has is that it is a munitions hog if you want to use it well, and USF always seems low on munitions.
The actual one in WWII was disliked by US troops. For some reason, instead of just copying the 6 pounder, the US changed the design and ended up with something that had much lower penetration and other design flaws.