"That heavy drum magazine, it was unreliable, heavy and like the drum magazine on the Thompson, it would come unlatched and fall on the ground just when you needed it the most."
For a drum, the PPSh mag was quite reliable (it was based on the turbo-reliable Finnish Suomi design), and it gave quite the reserve of firepower, which was appreciated.
But, it was heavy, and it was complex to manufacture.
Later in the war it was common practice for Russian soldiers to go into battle with the drum mounted, and once it was dry discard it and go to 30 round stick magazines that had originally been designed for the PPS 42.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.