Why get grenades when you can use a mortar and get faster weapon upgrades? Especially now that grenades were directly nerfed versus suppression.
I think factions should have clear cut weaknesses. Elite mod did the same crap where it added units for the sake of variety even when the factions had perfectly fine existing ways to fill that role. I'd be fine with it if the mortar was doctrinal though.
US lacks indirect fire in the early game because of how great riflemen are against other infantry. It's not like US was underperforming versus machine guns, they just had to deal with them through flanking or grenades.
Because going mortar reduces the number of rifles you can attack with (from 3 to 2), so if you do get a breakthrough you have less capping power. You also must have the mortar with your rifles at all times to get smoke, bringing smoke with you guarantees that you can always have it.
US was underperforming on city heavy maps where a single unit in a building meant that you had to immediately pick a doc with flames or get nades and burn away your munitions. This helps to alleviate this difference in strength by providing a natural, albeit light, counter to players sitting in buildings and stifling USF attacks early with little thought.
The extremely static build order from USF was also bad, because an opponent could know prior to the first mouse click the USF build order into the 4th minute. This should not be part of the game, you should have to make choices at all points in the game.
USF still lacks indirect fire, the mortar, as you should have noted from player feedback, is unlikely to be satisfactory in the role of bombarding an MG to death and getting more than one is probably near impossible in any game mode.
I understand that you are upset with the way the game is progressing, but I am afraid you need to accept that and decide if changes within that framework are worth your input. If not, I suggest at this point that COH2 is no longer the game for you.