The inaccuracy and range of Battletech artillery. A US Civil War battery has better accuracy then a group of Long Toms, unless you use Copperhead rounds. In a universe dominated by space travel, rocket assisted artillery shells and ballistic computers apparently became lostech during the 22md century.
Infantry weapons. Somehow 28 troopers with 5.56 assault rifles can blow apart a 1/2 ton of multilayered titanium, steel, diamond backed, cubic borom nitride armor, backed by another layer of honeycombed titanium that's designed for vacuum or underwater operations off a mech. That's silly. I can see sensor arrays or comm systems potentially getting damaged by standard steel jacketed bullets, but armor? Sigh. If's there anything that BT needs help with, it's infantry.