It is mostly the E8 and T34/85 I am talking about, and T34/76 spam with Windustry - not as big a problem as it used to be but, still wins games.
3CP+ Infantry call in's could be tied to T2/3 depending on what they are as well, but they are a lot less broken and counter's can be out in time for them.
@Coh2player, I've noticed that actually, the StuG does a decent job vs all medium tanks if you keep it at range and micro it. Shame you have to do the same thing with the Panther and at a much higher cost/worst performance

Jagdpanzer is similar, but I think Stug has better accuracy and damage? *Not looked in to the JagdP stats much so I can't confirm* Again as you said they are probably undercosted by about 15~ Fuel each, but that could be tied in to giving the call in a tech level requirement and keeping call in at it's current cost. Or simply by increasing the cost of the call in.
I have a feeling Fuel Caches only make this problem worse. Especially vs Okw. A "Fuel starved" faction vs a faction with 100% income + extra optional income for no downside? 400MP for 2 caches for an extra 6 fuel a minute is not a "downside" if you are good with your troops and don't bleed too much. You usually see 3 or 4 as well, so an extra +9/+12 fuel on top of regular income. It Seems a little wrong?