After months of cycling, the continued rubbing against plastic pannier clips has worn divots into the aluminium. If the rack were to snap, it will be at these weakened points. Tourer-talk suggests wrapping a strip of coke can under electrical tape at the contact points. In the meantime, both front and rear racks remain solid and sturdy, though other cyclists speak unkindly of wobbly low-riders. Maintaining a high air pressure greatly improves 'Conty' life and keeps them running fast.
Experience shows side walls are first to go, while the tread remains strong.
I interchange the front and back every once in a while to extend their life, and have Panaracer 'puncture proof' lining. So far, so good. For rougher conditions, I carry a spare folding Michelin 'Wildgripper'. Good for hard-pack surfaces and even rugged roads. It wears quickly, unless kept well away from bitumen.