Thing about tank destroyers is they had 1/2 inch armor around most of it. They only kept small arms fire from killing the troops inside it. A .50cal would make swiss cheese of it.
Considering the whines we get now about "My TIGR died in 1 hit!!!!!" imagine the complaints of M-18 drivers...
"I never even got to the fight! I got vulched by a P-40B!! 10 times!"