I think the ZiS is fine--it just faces tanks with far too much health and armor.
they need a faster setup time. and fix their microissues such as not being able to shoot over a bush.
they're actually OK vs armor now, the problem is just that there still is no reason to build paks vs armor when you can instead build armor yourself. however, their ability to deal with light vehicles is simply terrible and that cockblocks quite a number of interesting strategies - especially for the soviets. paks should perform vs. light vehicles like HMGs vs infantry.