That is the price for absurdly good vet and passives.
Increased range, from 60% rec acc to 60% acc bonuses, passive healing, permasprint, suppression.
Every single infantry squad have something unique and amazing above vet3.
At vet3 they have similar bonuses as other vet3 infantry. Everything above that is a huge bonus.
You can't expect to get that kind of advantages quickly.
I see you are cherry picking the vets that are better. Because most vet 4-5 are not. the price of ridiculous strong vet however is reduced ammo and fuel income. If its was a faction that has such strong vet but no fuel/ammo reduction you might have a point.
Basically you are saying however that the okw should just receive an ammo/fuel reduction for no reason because vet 4-5 are basically unattainable most of the time. As i said before reaching vet 5 for the okw should be parallel with vet 3 of the other factions.
And even then it will still take a lot of work to get most okw units to vet 5 even if that happened.