Humanity's scariest weapons aren't their WMDs. It's their precision weapons.

WMDs are a threat humans almost never employ, but precision weaponry are used in damn near every fight a human ship might get into.

Precision weapons don't do an extraordinary amount of damage; instead, they're EXTREMELY accurate. If there's the smallest gap in your armor and/or shield coverage, a human warship can put a shot right through it. In battle, human warships will routinely disable engines and weapons of enemy ships without killing a single enemy combatant, which is particularly humiliating for warrior races that consider dying in battle to be glorious.

When humans fight humans, their weapons are not nearly so precise. But that's usually because human ships shoot at each other from such long distances that such precision is impossible even for them, ranges that every other race would consider "out of range" for their own weaponry.