The Panther is fine; the tech vs call-in structure is fucked up.
If you compare the panther to any other tank, you will find that it is fine for its cost. It has both good AT firepower and high range and good that make it a unit that you can get a lot out of with good micro. A panther can solo any soviet medium tank (how well they solo depends on your ability to use the 10 range advantage) and it can put the hurt on Soviet heavies.
The teching cost isn't even that much of a problem vs soviet stock units. Against Soviet T4 you have plenty of time to tech up because Soviet T4 is not something that requires a immediate response. Against Soviet T3 it is trickier, but once you have a panther it is a big problem for soviet T3 units.
The problem with the panther is that this game has a cancer, and that cancer is the call-in mechanic. A panther is cost effective against a T34/85 or sherman, but it's not cost effective when the panther is outnumbered 2-1 right of the bat because those call-ins don't require a fuel investment for tech. Likewise a panther is not cost effective when you could just get a tiger for less resources.
If the no-tech call-in cancer gets fixed, the panther will automatically be a good unit again.
