Grenades are too easy to avoid by microing a squad in and out of the building unless the building happens to have just one exit and you happen to attack from right direction to be able to toss the grenade in front of the exit. This can cause USF to struggle against early garrisons (in 4v4 early mortar isn't necessarily an option if you want to fight back the early rush).
Note that the reason why OKW got fire grenades instead of regular grenades was because they are too easy to avoid by simple micro.
Flamethrower shouldn't be restricted to commander and should be accessible to rear echelon troops by default.
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