Penal Battalion have a ridiculous damage output from their rifles - like 3x conscripts. Plus flamer upgrade. Plus Satchels rule, as mentioned. Plus you can reinforce them for the same price as a conscript using Merge and they incur no deficit unlike other squads.
The German AA is also of less practical use because the Soviet plane strikes don't linger as long.
Seems like what you are really observing is a weakness in Soviet AA specifically. Quad does a good job. Perhaps there could be something else but it's workable the way it is.
Plus all Plane strikes are doctrine specific and you definitely don't want a situation where it's like "oh he got a quad, now all my strikes are useless cause it will just shoot them down".