On a 1911, during the feeding cycle, the cartridges do impact directly on the frame, and there is a large, square, sharp-cornered cutout in the frame at the rear of the slide stop. On my Colt Combat Commander the top of the cutout is open, whereas on my SIG GSR the frame rail continues over it. This would in theory make it more susceptible to stress cracks. But, especially with modern alloys, I think it would take an immense amount of shooting to cause any problem, if it even surfaces. Also, SIG frames are not immune to cracking. There have been cases of the frame rails cracking off, again after extremely hard use. However, they do still continue to function although accuracy is compromised.
I do not (yet) own a alloy-frame 1911, but concerns about frame durability are not even on my "radar."