[Suhiir]

Combat effectiveness is at it's base a factor of morale. And morale is made of a number of factors, some being...

Training
Dedication to their cause
Knowledge of what your opponent will do to you if you lose
Loyalty to your squad mates

If you look thru history you can find numerous examples where even one of these factors gave one side an advantage. And others where the same factor causes one side to route after superficial contact with their opponent.

WinSPMBT gives each nation/OOB a Training and Morale level, which can be, and in many cases is. altered decade by decade. In addition certain formations can be given modifiers (positive or negative) to the national defaults. Older versions of the game even had a "breaking point" where a sides moral collapsed and they broke easier and were harder to rally once a certain level of losses was reached (and I for one rather wish it still existed - it's annoying to have to kill every single unit or wait for the turn limit to bring a scenario to and end).

Ammo usage really tends to be a factor of training and availability.
Western nations tend to have superior logistics thus can afford to use more ammo. But take something like the SAS or SEALs and they can produce amazing results with very little ammo, they're trained to use their weapons efficiently AND know they have the ammo they're carrying and aren't likely to get a resupply.