I don't watch tournaments and "pro" players. I believe you that SU-76 lacks a role, that's why it should get a better barrage or sth. In my theory, it would be viable. It seems logical that the original relic team thought it out like that... maybe, don't know. Right now it's role lacking due to the fact that P4s are really early and T70 is pretty powerful in good hands, albeit needs to play it safe if raketen and paks are on the field.
I did have some luck fielding SU-76 with conscripts in custom games, although the opponents weren't really skilled.
I might be wrong and maybe SU76 is useless right now but maybe, just maybe, the role hasn't been found. In theory, skipping maxims and zis to go for SU-76 with conscripts should in theory be viable, especially with the fast snare on conscripts. That's all in theory. Maybe somebody goes YOLO and builds SU76 and wins a game and shows that the SU76 has a role.
A lot of "META" was found. It was not straightaway clear from the patch notes. "This unit is getting these buffs" and then people go "Oh this unit was buffed, let's play around it", and hence a meta arrives. I mean, I've played Dota2 and often enough my experiments ended up in clear wins (top ranks in dota2). I don't believe that in a game with so many dynamic variables, there exists only one build order and few METAs. Maybe there exists a SU76 build that can be really viable vs distinct OST or OKW build orders. Who knows. I admittedly, don't
I mean we've had the issue of the SU76 already before, so I'll make my reply quick:
In my eyes, it lacks a role. It's stats seem fine for the most part, also compared to the StuG and the Puma. But you don't need the SU76 as Soviets.
If we look at the early-mid game in a somewhat even match, the worst LV you can face is the Luchs. But your T70 is already better than it and can beat the Luchs, so you're better off with the T70 since you also get excellent AI power. Similar reasoning for most things you could face: 222? 251? MG spam? Mortar spam? -> T70, the SU76 won't help you because it's barrage is relatively costly.
The only unit it could help you against is a Puma, but a semi decent OKW player won't build the Puma if you did not get a T70 beforehand.
And for the mid game, you always have the better option of the future-proof SU85. If you play T2 an need more AT, you're always better off with investing that bit of MP more into the ZiS and save up the fuel for the T34 or SU85 instead. If you play T1, back teching is often better as well since you will likely need an ATG for the late game anyway.
The SU76 is - as all AT units - a reactive unit that you don't buy proactively, but only when you need it. For the LV phase the T70 has all the AT you need. For the medium+ phase SOV has more reliable options. Additionally, the SU76 won't secure you points and support like a T70 does to get back the resource investment. And to top it off, SOV have to make this AI/AT choice in a phase where Penals have lost their initial shock value, Conscripts have not reached their 7-men plateau yet and Axis infantry is upgraded.