I have not had a rack fail and come off but I have heard of it happening to several people. I know some of the older Thule towers had a sleeve part that the bar went through that was attached to the foot of the tower by nothing more than a pair of aluminum rivets.
I too have found that triangulated lines running from the forward stem of the canoe to points at either side of the front bumper, or to short nylon straps mounted either side of the hood for those vehicles lacking suitable anchor points below the bumper, do indeed reduce yaw induced by cross winds and semis considerably.
I don't find that triangulated lines at the rear add that much and don't routinely use them. I usually bring a painter mounted to the rear stem of the boat forward and loop it over a thwart, then run it back and tie it to one of the crossbars behind the thwart. This very effectively prevents the canoe from sliding forward during a panic stop. More effectively than lines running from the rear bumper to a thwart, since invariably some slack must be taken up in such lines before the canoe stops sliding forward.
Pull-Up Strap Handle Kit
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