They were identical to PGs with the STG upgrade with slightly lower target size and flipped vet2 and 3 bonuses. What made and would make the difference is the tactical advance ability on them.
That's exactly the issue, with STGs they are nearly identical to PGs, and that isn't good for unit identity, but it's good riddance too, with STGs you could easily park and hide them on your enemy's retreat path, pop tactical advance on any retreating infantry and boom, you just got an easy squad wipe without too much effort, tactical advance is too good of an ability to be allowed on a weapon that's decent at all ranges.