The biggest issue is that Ost has a cheaper, more cost effective tank hunter found literally a phase below it. Which has one of the highest DPS's in the game and is very cheap.
Panther and Stug has a similar issue with the su76 and Su85 if you multiplied the problem by ten fold. It makes the Panther not worth it in most cases.
Nondoc Heavies are bad IMO. Just look at the mess the KT causes to balance.
With alteration to SU-85, StuG has THE highest TD DPS in whole game.
Only vet3 puma compares.
And the only vehicle StuG is not very cost effective against is IS-2, which 2-3 StuGs will still make a short work of.
But yeah, amazing-ness of stug does overshadow panther by a long shot.
Panther is there to counter KV-1(literally anything can beat KV-2 since it was made into bigger dozer that can't hit a broad side of barn), comet and IS-2 and nothing else really.