jonnybigguns, the problem with the S&W M66 is that it allegedly wasn't really designed for .357. It was designed for .38. All the M66 is, is a Model 10 that was stretched slightly to fit .357 in. Supposedly, heat treat was also different, too. In order to get the gun as small as it was for the caliber, the bottom of the forcing cone had to be milled off to make room for the crane (or yoke or whatever S&W calls it) to close. This results in a weak spot on the forcing cone where it was milled away.
Allegedly, as long as you don't use light bullet weight .357 loads (like the 125 and 110 gr loads), the M66 was good to go...for a while. Allegedly, even with the heavier bullet loads, M66s shot loose after a moderately steady diet of full-house .357 ammo. Also allegedly, the M66 was designed in a time when you practiced with .38 and carried .357, or so the story goes. This was why it was designed the way it was.
With the GP100 and M686, the guns were designed from the ground up to shoot the lighter, faster, more violent .357 loads. While there is healthy debate on which is actually stronger, it would take a LOT of shooting full-house .357 loads through either of them for them to wear to any appreciable extent. They are just larger, stronger guns. Shoot anything you want through your GP100. I'd bet they you will wear out before the GP wears out!