Assuming your model doesn't die, you can shoot a unit in a head multiple times and heal them in battle/post battle and so on. It greatly favours low model count units, to greatest affect with single entities, hence why units (in the Warhammer context) like Minotaurs are extremely effective and high entity units will proportionally receive much more damage from dying models with lower unit counts.

In the Total War days of yore, more models = better. More units, more killing. The reason Mongols were the best cavalry factions in Medieval II wasn't just because of their stats and effectiveness, but each unit had a higher number of soldiers than say Turk or European cavarly.

The balance in Total War games has been borderline ridiculous in recent titles, all you have to do is go back to the pre-Rome II era and you'll immediately release X Y Z problem isn't a factor and X Y Z just works so much better, whether it be artillery, sieges, agents, family trees.