Scaling doesn't necessarily mean vet. Weapon upgrade (for that sweet dps concentration and dps boost through munition), nades, snares, AT, resilient to late game wipes, utility, cover creation, reinforce cost, reinforce speed, etc.
Penals are a great option early mid and fall flat on the later stages of the game, specially if you have to rebuild them. Conscripts are the completely opposite of them. PF are basically a middle ground between those two.
I tested Penal fighting power todaz vs Grens and also a bit vs Volks.
They do very well, win most fights with ~3 models (sometimes 2, a little less often also 4) both in and out of cover at least when at range 20+ (have not tested below that except for running Volks into Penals from range 30). I tested vanilla Penals vs Volks/Gren vet 0, vet 3 and with Volks/Gren weapon upgrade at vet 0 and 3.
I think Penal scaling criticism might come down partially to over exaggeration. The other part are true factors like lack in utility/reinforce cost etc.