Kubelwagons often force your opponent to spend resources on tech they otherwise wouldn't. Soviets can panic build scout cars or AT grenades, Brits can build a bren carrier, USF may pick mech doctrine or rush for an M20.
So salvage their light vehicles, or your own dead kubels. (Although they really shouldn't be dying to such things.)
By the time he has flamers you should have a Luchs/oversoldaten. I go mech --> Panzer --> battlegroup, only using mech to convert resources as needed until my panzer command is up. This lets me get a Luchs on the field at around the same time as a T70. Wreaks absolute havoc on USF because they are so very rarely prepared for light vehicles.
Kubel is useful only on certain maps. And any decent player can work around it by using green cover/garrison. It's not an MG42, so it won't do as much damage. You don't have to panic retreat if you are suppressed.
I sense that you think mech is the way to go as OKW. I'm sure if that would be the case, we would had seen more of it during OCF or from any of the other top players.
Going mech > Flak HQ on 1v1 puts you way behind on the attrition war and makes your fight sloppy as you can't fight back at 100% health. This would be feasible if Sturm vet1 medkit was improved or reworked.
If you wreck a USF player with a P2, then they are doing something horrible wrong. Either they get a Cpt and a Stuart (which hardcounters your P2) or they should had either tech major/Cpt if they had gone for M20 play.
Most people skip major if they don't have a big edge on you to pull out a Sherman before they are punished by no AT.