You amuse me, please explain to me how a faction with the cheapest mainline infantry with vet5/shreks and a cheap at gun in t1 along with pumas struggle vs scout cars. Soviet t3/4 isn't going to beat a experienced Okw opponent hence why everyone use call ins and sovs don't have any effective mainline infantry making maxims and snipers the only thing Soviets make every game because nothing else is as effective, and Okw has no counter to shocks what crack are you smoking? That's the dumbest thing I've read yet, that's not to mention how stukas ruin any t2 play and potentially snipers. Okw isn't a weak faction.
For the M3s: timing is everything.
Your flamer + m3 hits WELL before the first schreck, let alone the first 251 flak or puma. Even 1 schreck isn't enough to keep the car in check. The schreck can only be in one place, the car has a huge mobility advantage. Any lone squads are going to get picked off/harassed/bled, forcing you to play mainly around your lone schreck squad - and god forbid you lose it.
Every game that I open with M3s I get at least one squad wipe, generally more. Plus it gives CP and vet to either penals or your engineers. Vet 2 flamer penals are super hard for OKW to deal with because Oorah lets them soft-counter Obers while completely hard-countering Volks/Sturms.
I doubt I'd open this way in the current 1v1 meta, because the 251 flak is so strong against T1 openings, however I almost always open this way in 2v2s, and have my USF partner get Capt. Capt tech utterly dominates OKW Mech openers. In my opinion it is better to open 3-4 Cons into T2 as Soviet vs OKW in 1v1.
T3 and T4 are both quite strong against OKW, again because of the timing. If you have map control going T3 for a quick T70 or T34 can be extremely punishing as OKW doesn't have access to PaKs and Raketens just suck. Their Panther is going to hit long after your initial vehicle, and you'll have either ZiS guns or Jacksons to contend with the panther anyway. If you don't have map control, you skip the tech and fall back on your IS2s or T34/85s. T3 is going to be better in 1v1 than in a 2v2, because your USF partner is probably going to rush a Sherman anyways, there isn't much need for your T3 tank.
Shocks are super good against OKW because of the way all their weapon profiles work. You can close in on Volks extremely easily, you dominate Sturms, and with proper flanking you can do well against Obers. Of course you'll lose to the Obers if you try to run straight at them but with green cover/true sight/smoke it isn't that hard to get into close range, at which point you absolutely wreck them. Biggest issue for Shocks is the Luchs/251 Flak really. But neither of those units is scary for a Con/T2 opener.
Finally, OKW has literally no good answer for Is-2s - and tends to struggle against T34/85s. The Panther can keep the IS-2 at bay, but struggles to ever kill it, especially when you have to fear AT nades/mines/ZiS constantly. Meanwhile the IS-2 is running around 1 shotting squads, laughing at schrecks and getting to vet 2... at which point your Panther is absolutely gardened.
Not to mention, you literally cannot open with a forward med truck against Soviet. You will lose to 120s every time.
If you read my post more carefully (reading comprehension is important boys!) you'll see that I never called OKW a weak faction. In fact I clearly pointed out some issues where they could be toned down/redesigned. They're undeniably the stronger Axis faction but they're absolutely disadvantaged against Soviets in 1v1 and 2v2.