ROF for TD hardly means anything without penetration. Firefly has a garbage ROF but its still very usefull and powerfull TD simply because of its raw penetration, its less effective against medium armor while being effective against heavy targers.
Stug and JP4 have good ROF but very poor penetration, meaning that they are effective against medium targers, but they are slightly better then premium mediums against heavy targets. Stug is cheap counter to mediums, while JP4 is somewhat counter for allied TDs and mediums, but it hardly can be massed, so its more of a single support unit.
Point is being, when it comes to TDs axis do have problems vs heavy armor, since aside from AT walls, the only thing that can reliably fight heavy armor is panther.
Honestly I never understood this design, why would Axis get TDs which main goal is to couter mediums, while Axis objectively already has so many tools to counter mediums, while against heavyes they have no other reliable option but panther.
Not to mention that Allies can potentually get more heavy tanks and faster in general then Axis.
Yes, their penetration is lower compared to ally TDs but also take into consideration that Stugs and JP4s go against Shermans, T34s and Cromwells.
Stug/JP4 pen: 200,185,170 VS
British medium armour:
- Cromwell 160
- Comet 290
- Churchill 240
- Centaur
As you can see, Stug and JP4 can completely deny British medium armour except Comet. For Comet, Panther is more than enough and in every scenario it wins. Stug will penetrate Comet 60% of times, which can be enough with RNG in your favour but is still not enough to rely on it. Churchill is pretty much a sponge which can easily be used as veterancy gain for Panther/TDs
USF medium armour
- Sherman 160
- E8 215
- Pershing 270
- Bulldozer 200
Again, Stug and JP4 can deal with all ally armour. Against Pershing Stugs and JP4 will not penetrate most of the time. Again, Panther will easily win vs Pershing 1v1 and in typical scenarios with supporting infantry, it will force the Pershing away.
Soviet medium armour
100% Penetration against everything except KV"X", ISU152, IS2 which are all doctrinal heavy tanks. Still, Panther wins against every KV# tank and wins vs ISU152 and IS2. Only problem it can have vs ISU152 is the range, so charging it head on is not advised (same as vs Jagd and Ele).
Moreover, JP4 has 230 frontal armour but no turret so it evens out if an enemy Sherman or T34-85/76 or Cromwell is charging it. They won't penetrate most of the time and with the target size of 17, they won't really hit reliably on the move.
So, you are right that Axis have a lot to counter mediums but it's because mediums is what you will be going against most of the time.
USF only has one commander with Pershing and you can use JP4 and 2x Stug to force Pershing away. 62% is more than enough since the Pershing has 960 hp, same as Panther, so no sane person will risk having it damaged only to be repaired for a long time.
Soviets on the other hand have plenty of heavy tank commanders but all of those tanks wet themselves if going against a Panther. And all of them are slow enough for Pak40 to be more than enough in conjunction with Stug or Raketen with JP4. Combined arms. ISU152 needs a rear armour nerf but that's another problem.
Ally TDs will mostly go against 260 armour Panther or f***ton armour KT or Tiger commanders or the popular Sturmtiger commander. Also Brummbar with it's large hp/armour pool and OKW P4 with it's 230 frontal armour.
See the disparity and the need for 60 range TDs on ally side?