If you have to hard-cap a unit it means the unit isn't balanced in it's cost in the first place, and you are just imposing an artificial means to make it look less OP than it really is.
I don't fully agree with that statement. Things can be balanced as long as they are limited, look at the king tiger. A company having one of them is fine as the enemy can counter it, but more is just gay. Soft-caps are an excellent way of balancing things, though if someone can effectively by pass it there is something wrong with "soft-cap" implementation not the idea behind it.