Brutal Momentum does not alter the maximum number of targets you can damage. It will allow you to (theoretically) hit an unlimited number of enemies, but the number of enemies who suffer damage is constant - that is controlled by the weapon’s damage profile, which specifies how much damage/impact the 1st, 2nd, 3rd (etc) target will suffer. Some Fatshark employees have explained it in a bit more detail in this thread, if you want additional info.
In short, Brutal Momentum isn’t very good on the shovel because, by design, the shovel can’t damage a lot of targets.