It used to have a weaker barrage, but no cost for it.
It was well-balanced for 1v1, and sometimes people would go for it instead of the T-70.
However, in teamgames you could spam the things. The SU-76 blob could take on most enemy vehicles with sheer volume of fire, and the stacked barrage would annihilate any infantry AT sent in to deal with them.
I'll argue it was not well balanced for 1v1 as well. We would see the Su76 blob on maps like Crossing or Langres.
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Ding ding ding. We have the best answer.
There's no room for an Su76 on an equal match. Cause on an equal match you would want a T70 as you should be behind after spending mp on AT nades, medics, a Zis gun (or reducing your AI even further with PTRS) and finally teching T3.
The problem with the Su76 is that it just delays your tech. If you need AT you are better off with a Zis gun. And it's not like you can just stall with Su76s into call in tanks.
The only weird scenarios on which i see the Su76 been the correct call, are against rushed Ostwinds or prolonged mechanized play from OKW with Stuka been involved.