You forgot how OP their veterancy is. And they actually manage vet 5 quite often. With permanent sprint. On your whole blob. You do get it now, don't you? Its like having a whole army of USF officers. Not to mention that fusiliers actually have AT snare, while neither officers nor penals have one.
Btw who asks for penal buffs? Incoming patch is nerfing them in multiple ways. Some people just wanted to have something instead of the flamer that is not as game breaking.
Drifting of topic now:
PF are great at vet 5 I will not argue against but, at vet 3 Penal are better the PF.
Penal hit the field earlier and have lower XP Value so they tend to vet faster.
Penal get a snare in march Patch
There is weird theory going around that unit without weapon upgrade become "irreverent in late game" or "do no scale well into late game."
https://community.companyofheroes.com/discussion/243778/soviet-infantry-lineup-doesnt-work-ussr#latest
But there is little point debating PF vs Penal in this thread...