Hmmm...my GWACS lower would like to disagree with you. As would Ian and Carl of InRange with their WWSD AR-15s who also use GWACS lowers. Granted mine hasn’t had more than maybe 200rnds through it, but they shot, run over, done pushups on and fired (I assume) thousands of rounds through their GWACS based AR-15s with no breakages. Uses polymer buffer tube integrated with the lower. Much tougher design than polymer lowers with a screw in tube. Stronger than aluminum lowers with an aluminum tube (you’ll bend the tube if you run it over with a car. Carl and Ian tested both side by side).
I can’t personally speak to SIGs design. It might be fine, it might be crap. Many firearms are starting to use polymers in parts of their actions. I wouldn’t use a firearm I didn’t trust for possible life or death situations. But on the other hand, you don’t know that that rifle is less reliable.
My garand’s action is nice and exposed all the time, especially after ejecting a clip. Worked great for 20+ years in US mil service and longer for other countries. Yes it can lock-up if exposed to a bunch of mud, ice or sand.
Doesn’t seem likely in an HD scenario. DA revolvers don’t work if the bad guy grabs and holds the cylinder. A browning action pistol won’t fire if the barrel is pressed up on something pushing it out of battery.
