Eh, fighters are capable of dropping 6d12+96 damage at level 10. Assuming a 60 percent hit rate they're dropping an average of 90 damage, and a max of 168. They aren't underpowered, they're just forced into GWM/Sharpshooter and they don't get magical defenses without help. Which are big mechanical problems but not really power issues.

You call taking 7 attack actions in a turn "underpowered"?