Actually I can write novels about penal legions, because I have played them very very often.
Lets try a novel
OK are they good?
Its hard to say, because you cannot say, they are always good. The problem is, that they need a few extras to excel in battle.
And you have to BE the penal legion to use them properly (in other words, no mistakes allowed!)
First point:
1 penal legion is something about nothing. Its basically an infantry squad with a random extra which is stubborn and easily taken out with little anti person fire. 80p? Not really a good investation.
2 penal legions are 2 infantry squads. The enemy will have to shoot down 20 men, which is far more complicated, so you can allow at least one squad to make it to target.
3 penal legions are in my opinion top choice. Why? One penal legion for each extra. So you have a good chance of getting at least 1x the certain extra you want in this game and you can use the others as meatshield (for that unit or your main battleline). And 1 unit of knife fighters may not be able to beat
meq squads, but 2 knife fighter squads can make it.
Second point:
They do need support. So you have to use a storm troopers chimera with meltaguns at your flank to prepare the enemy target for the charge, to cover the march of the legion, to shield them against dangerous opponents. On their own on the flanks penal legions will be killed soon by decisive countercharges.
Third point:
How do you use them properly?
Well first, you have to know what they are able to, and what they are not able to. Dealing damage to anything is not their job at all. They are best used marching around, where no enemy is, but where the objective is. They sneak around behind terrain just to march on the unguarded objective. That is a job a normal infantry squad cannot do because it has to punch through the enemys army to get the objectives that are far away. In case of the legions the enemy has the bad decision to turn around under constant fire to take down the legion which is guarded by a countering storm troopers unit or to continue the assault with the risk of losing an important homebase.
Thats their use concerning objective missions.
But you can make some other interesting stuff with them.
There are some killakoptaz heading for your tanks for a first turn kill? Alright. put the penals in front the tanks behind, the koptaz must keep 12" from your penals, if you win the roll off you spread out forward to limit the range of the koptaz. Same to infiltrating
cc units. put them in front, wait for the infils an then step back.
The enemy has outflanking units? no problem. Take 3 penal legions and close one flank and deny the other. You dont have to bother about them anymore.
Conclusion:
The best fitting attribute for a penal legion is in my opinion: tricky.
For you to use them properly and for the enemy to deal with them correctly. Their biggest edge is, that they can react very well on the enemys actions.
Hard facts normally speak for the infantry platoon, but the penals have many soft skills that no other
IG unit has. The most important: Threatening the enemys back without punching through their army.