The frame should have some flex, regardless if it is a S&W, Glock, H&K, Ruger, etc. It is polymer and polymer flexes. That flex give you durability, absorbs recoils and prevents cracks. Guns with too stiff of polymer (or zytel or nylon) that do not have enough flex will crack and break during recoil. This was what lead to the demize of the Ruger P95 series that used a glas filled zytel with molded frame rails (which had a tendency to sheer off). There are videos of the FN FNX series looking like soft rubber in the hot Texas sun, but the gun still shot just fine.
I have polymer framed guns by most makers, the Walther is the stiffest and the Ruger seems the least stiff to me. I am not at all worried about any of them, especially which one I can make appear to flex more or less or how that may or may not apply to the quality of the gun. That being said, I have lots of options when I go out to the range or decide what to put in a holster each and every day; including 3 different generations of Glock 19 and performance center M&P's. Often I choose the SD9. I just like the freakin' thing.
