Undoubly penals are strong, they are however pretty forgiving in terms of wiping power. You can more easely play around them cuz of their slow start, you halt them with the mg42 more easely.
Pgrens however are much more leathal overall, dps is higher, their nade much much stronger.
It is a bad design regardless. One can delay all these strong unit or have them start less powerful and give them weapon upgrades.
What does not work is having PG later and facing vetted infatry. Before the patch in a standard 3 grenadier built PG being the 4 unit was simply unpopular and for good reason.