Flame nade = repeated munitions cost. A squad can leave a garrison and move to another or to cover. OKW squads CANNOT do this vs flamers.
ISG = very slow to hurt garrisoned units. Penal flamethrower = very fast.
Stuka = 100 fuel and comes much later than penal flamethrowers.
The Soviets have a great advantage from the very early game, which translates into a fuel advantage for them and a fuel lack for the OKW (slowing down your Stuka solution)
Lastly: (even though you completely changed topic)
"usf have to face garrisoned (the most op mg in game) MG42"
Vickers is arguably the best MG in the game. Gets more range when vetted and does more damage.
USF also have smoke grenades on their mainline infantry for a small fuel cost.
Again, smoke won't kill garrisoned unit.
For USF, it is even more difficult to deal with garrisoned unit as most of its methods to deal with garrisoned are more expensive and less effective when comparing to OKW.
USF: MKII grenade= extra side tech cost(Flame nade come with tech) and can't even force the garrisoned unit to get out if the building is large enough(Flame nade can!). Even multiple grenades are thrown, it has a 1.5 second timer for opponent to react (Flame nade doesn't!). And quote from you "A squad can leave a garrison and move to another or to cover." when facing MKII grenade too.
Conclusion: OKW's Flame nade win!
Pack howitzer =
Even more slow to hurt garrisoned units while being more expensive than ISG with less range.
Conclusion: ISG win!
M8 Scott =
65 fuel(Directly rush to M8 scott is around 250fuel while stuka is only 160 fuel) and comes
much later than stuka. While stuka can one-shot most of the building without the need to pray for RNGJESUS on your side while usf doesn't have any non-doctrinal mobile artillery option, balance?
Conclusion: Stuka win!(Comes earlier and more reliable than any mobile artillery of ally)
Summary: OKW has a much easier life when deal with garrisoned units and they have enough options already.
If any changes should be made, USF should be given more anti-garrisoned ability.