I'll just add my experience to the 'debate' FWIW!
I have 4 x 591 racks which have been used with at least 7 different bikes ranging from a tiny kids bike with stabilisers through various sizes of kids bikes upto assorted fullsize adult bikes (MTB's/hybrids/roadbikes). Some of these have, due to size, been clamped in a less than optimal fashion both to the frame and at the wheel. Despite this they have travelled fair distances at enthusiastic speeds on motorways and twisty A roads without ever giving a hint of coming detached. Clamps have ALWAYS been as tight when unloading as when loading in the first place.
My gut feeling is user error. My only caveat to that is that
Evans have not actually bothered to send the rack to Thule because in the slim chance that the clamp was really faulty I don't for a moment believe that a reputable company like Thule would allow the product back out into the real world!