The M42 has range advantage.
You can fire at light vehicles from behind a screen of AT nade wielding conscripts with the M42 without exposing the unit to enemy fire. A guard squad has to be directly on the front-line to fight, vulnerable to repressive- or focus fire from the enemy. This may force an early retreat of your AT unit, this collapsing your front line. The M42 does not have this limitation because it has range.
On the other hand it has also cons. Can be flanked, can be circled to death, can't retreat etc.
Everything has pros and cons.