Are you fucking kidding me? you are saying that an unit should have no counters when it arrives completely able to rampage in your lines with impunity. I think you are the one not thinking straight here.
Oh yes try to beat the m20 or even better the flak track with the raketten or shrek thats fun stuff and a good way of losing the game.
No, I'm not saying that. But there should be a window of opportunity. And the M20 is easily killed by raketen + schreck combo. If not killed, it's ability to "rampage" is severely hindered. For the flak HT you have the puma. And schrecks + raketen in buildings.
Oh, btw the Puma also makes soviet T3 totally useless since puma + schrecks counter it perfectly before you even have the T3 building up. Don't even really need raketen,they help of course.
Puma + schrecks also counter the Stuart before it's on the field. So if you choose captain against OKW, your only effective way is the AT guns to deal with armor. But going captain instead of major delays your own tanks a lot. So there are basically only bad choices against the OKW as the USF, and the puma isn't too bad (in combo with the schreck blobs again) against USF tanks either.