When shooting the XD, I found the grip safety to be awkward due to its small/thin profile. It was hard to tell, by feel, as to whether or not the safety was fully disengaged when shooting.
That has not been my experience. I find the grip safety on the XD MUCH more foolproof than the 1911 grip safety.
I have never even once had an XD fail to fire from an improper grip. I can't say that about 1911's.
Please define the context of "foolproof" here. The gun mechanism's design functionality or the shooter's ability to grip the weapon?
A grip safety, by design, prevents a gun from firing if you have improper or inadequate grip on the gun. If the gun doesn't fire because the shooter has an improper/inadequate grip... one might suggest that the grip safety mechanism is technically functioning.