They had some issue with the slide stop spring but as I understand it the newer guns come with a different version which prevents the spring from popping out. Here is a link with some pertinent reading. I believe it affected both the 9 and 40 platforms.I heard the .40 had some issues
It does seem like the Walther lines are very underrated. I never hear much about the PPQm2 either , and most of the reviews I've seen on it are very positive.The PPS is one of the best modern single-stacks on the market. Buy with confidence.
I find all the recent hype about the Shield and XDs-9 a little amusing, considering the PPS has been on the market for a few years, and is every bit as good as those pistols. It might be ever-so-slightly larger than them, but has the benefit of shooting like a MUCH larger gun.