I think the jist of what James is saying is that since Sig designed the .357 Sig round along with Federal, and hence did a tremendous amount of in-house testing with it before release, they had the inside track on designing guns for that round and have a deeper well on their intellectual database regarding it. I don't doubt that's true.
Today, after 22 years of it being on the market, I'm sure other manufacturers understand it pretty darn well also, but since it wasn't initially a caliber designed around thier own guns, they probably didn't do near the R&D that Sig did. I agree that Sig realized if a P229 or P226 with a stailness slide could handle a hot .40, it could with a few tweaks handle the hot .357 Sig. I don't think other makes are unsafe, I just think with Sig's name on it they may care a bit more about it working right and having longevity.
Personally, having shot the .357 Sig through Glocks, I would prefer to have a Sig when firing off that round, which may be the second point James was making. Frankly, I think the round is simply too much for any polymer striker fired platform because I just don't think they have the robust nature of a metal framed pistol. In 22 years of being an LEO Firearms Instructor, 99% of all "kB!'s" I've seen are polymer striker fired pistols (mainly Glocks) shooting either .40's, hot .45's or .357 Sig's. And when a polymer bodied striker fired does go BOOM, it goes right through the body of the pistol, which includes the portion serving as a grip, and into the shooter's hand.
The only Sig I've seen go kB! (and a Springfield 1911A1 10mm shooting hot loads) all blew the mag out and simply cracked the wooden grips or G10 Hogues. So yeah, I'm kind of with James that if you're going to shoot .357 Sig, you might be better off with a metal gun like a Sig that, due to the weight, can eat the recoil pretty easy, won't get battered and won't de-finger you if it goes go pop.
As far as performance, I understand the new FBI protocol and why many folks believe modern 9mm's are the equal of .40's and .357's. I simply think they're wrong because their manner of scoring is overweighted in bias for penetration, creating what I believe is unecessary penetration. I've shot the "new' style rounds against the older ones (.357 Sig, .357 Mag, hot lightweight 9mm) against steel and into meat, and despite what a certain overwrought, overfunded, overtly political, unaccountable federal bureaucracy says, transferring huge amounts of energy into a potentially murderous human is critical, and penetration is secondary. The old phrases of 'energy dump' and 'energy transfer' may be semantically incorrect but the concept of using more energy dynamically through faster, lighter rounds is no theory...it's been done for over 50 years on the streets with the aforementioned rounds and I simply do not believe that the Feds and Hornady have successfully repealed the laws of physics.
My agency is going to the Critical Defense 135 gr 9mm (simply because the FBI says the 9mm is now 'best'), but it'll take a lot of street results before I carry that round in lieu of the .357 Sig, which has been stellar in belt-buckle to belt-buckle incidents with our Troopers. Yeah, folks think it was designed with car doors and windshields in mind, but it has worked remarkably well in person to person calls, also.
It's a great concept and still valid today. What is interesting is that my agency, for probably a year or two, will have both on the street, so in essence they will be measured against each other in real time. Perhaps a 9mm can be designed that can replicate the .357 Sig's performance, or even exceed it, but for today and until a lot of supporting real world data comes in, I'm sticking with those proved Gold Dots.
But if you think it's worth it, I'll try. 
