Although a lot of grabrails aren't easy to hang onto I haven't heard of an accident from failure. If anything the mountings are over-engineered, no factory wants to fight that sort of product liability case.
However if a bike has been crashed and gone down hard on the grabrail the mounting points on the subframe could be damaged and weakened. Early Suzuki GSX-R750's and Honda CBR600's were prone to suffering like this.