I don't think the SU-76 is really that bad, it's a bit fragile sure but it's a decent mobile indirect fire platform that can also scare off a luch/flak.
It's decent at indirect-firing at a slightly farther range than an AT gun that costs no fuel. It can scare off vehicles that are nearly useless against other light vehicles while also being useless against anything that costs fuel else while both a Luchs and a Flak can fuck up anything on legs.
I'd take those two vehicles any day over an SU-76. Yes, I mean that example even if that means I have to face down tanks with only AT guns, at least I'd have the AI part of the game set for sure. If I have an SU-76, I expect nothing to be set for me.