I heard it thousands of times, yes t0 mortar with buffed dmg for garrisoned units i agree, but it leaves you with less rifleman, so holding off enemy charge at your mortar is harder. Another thing OST does against me is they simply build their own mortar, since they can at about the same time in their t1. What do you do then, huh? Remember that all the time fighting they have the fuel and are snowballing. :/
So what you are saying is that it's imbalanced that USF, instead of just building a rifleman blob, has to get a support unit because it's unfair that a machine gun is able to hold out ( as it's meant to do)
I hate to break it to you, you don't get to counter stuff for free.