I think the SU76 has a efficiency problem.
Against OST you don't buy a 75 FU unit to counter a 222 or 251. Against OKW you might use it against a Luchs. However, the SU76 is still more expensive. Also the T70 can scare a Luchs away if necessary and brings very good AI to the table. Against a Puma it's not worth building it, because it can be killed too easily if it gets caught out of position, while it itself can't dive the Puma.
So what is left for the SU76 in it's primary role as AT unit? Osteind. Would eork alright, but just get a T34 instead. Against StuG it does not work that well. Against P4 it's too unreliable so you'd need another AT unit anyway. It's not worth delaying your own tank that heavily.
So the only role that is left is if you're massivlely behind on furl, your opponent get's a tank out on the field and you can't afford teching to T4. Then your only hope is to get the SU76 and hope that you can somehow kill the P4 to narrow thee gap
Su-76 can take out most vehicles when in number.
According to test someone did (do not remember currently) they could take out an Elephant (3 or 4 if I remember correctly)