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.
How safe are grabrails, my girlfriend gets worried when I'm riding my Kawasaki ZZR600 quickly and all she has to hold on to is a size-inch strip. Does anybody know of any serious accidents caused by failures?