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)
But it's not cost effective anymore.
So I just tested the Elefant statement with 4 SU76:
Yes, it is true. 4 SU76 can kill it. At least two of them die, they take quite a while to kill the Elefant and they have to drive to super close range to somewhat reliably pen. And I tested in a stand-off. Since SU67 is a casemate I also tested for frontal assaults only (although with 4 of the units next to each other there will be side armor shots depending on which SU76 the AI Elefant targets.
Still you invest 4x75 = 300 FU (and lose at least 150 vs Elefant 245) and 4x280 = 1120 MP (lose 560 vs Elefant 720). Side note POP: Su76 need 32 vs 22 of Elefant.
But this is nothing that would happen in a real game. You'd just waste 1-2 SU76 and that's it. You must frontally assault into PaKs because flanking with casemates is very hard. You're probably better off with 3 T34s and doing a real flank to be honest. So while theoretically possible, it's not a real point for the SU76.
Now can this argument be transferred to other units? I think it will work better against a Tiger due to lower armor, but still you need to assault frontally into support units and you always have to go all-in, as the chance to penetrate at long range is about 55% which is probably not enough to deter a Tiger reliably. Instead of 2 SU76 you could also just get a SU85 at this point in the game that at least is slightly more resistant to mis-positioning compared to a SU76.
The SU76 is probably not an inherently bad unit, but there is just no unit worth countering. The SU76 is too expensive for the early game units it could counter. On the other hand it comes relatively early so it rightfully should not be able to counter late game units reliably.
The AI barrage just does not make up for it. SOV have better options at next to every point in the game than an SU76.