the tradeoff here is the lower DPS and extremely low health... the SU-76 dies to 3 shots from a medium tank while taking 6 shots to kill one... it can most certainly harrass P4s... maybe stugs if the enemy is a dumbass... but it can neither bully nor snipe these units...
the SU-76 is widely considered to be an inefficient unit anyways... its value is easily superseded by the SU-85/ZIS3 combo... its the reason why you dont see the SU-76 very often...
Ok. We can say SU76 and Stug are quite similar in terms of AT option.
BUT, the SU76 have an extra ability (Barrage).
The same happends with ZiS vs PAK.