well this is fairly RNG dependent. technically, even a flame engi could beat a pgren if you get lucky enough. the only thing that would make either of these units win is flame crits. that RNG could easily work against you too though. if the pgrens get lucky and blow up your flamer, the soviets definitely lose that fight.
I'm not going to concede that point. As far as RNG is concerned, Flamethrower Penal Battalions have the clear advantage unless of course the Pgrens land a grenade.
Pioneers can beat conscripts with RNG, but no one depends on that when they plan their strategy.