I've been using the stuff since it first appeared and have never found a way to reliably guarantee that it doesn't peen the case mouths. And once they are peened, it is a huge PITA to get rid of IF you need to. It won't affect lose chambers on pistols, sporting rifles or the like, but it causes HUGE headaches with precision rifles (custom chambers).
Someone said that it goes away during resizing. Nope! Not even with a Lee Collet dies... Still very detectable... the traces stick out like a sore thumb if I fire the cases through the Pressure Trace system (looks the same as when cases have donuts). Just pushing a bullet into a fired case, you can feel resistance. There shouldn't be any with a fire case...
Someone is going to say that it won't matter because it'll get trimmed off. Yeah, the first or second time you trim. However, the peening also shortens the cases, and once they stop stretching, they end up getting peened to less than trim-to-lenght and then it doesn't get trimmed off.
SS works great for some applications, probably most applications the average shooter will engage in. However, it isn't the right solution for brass that you're going to use in something like F-class. US is the way to go for that application.