Sort part failure of a tire blowout is really the only thing I worry about sometimes. I have a suspicion that inner tubes can deteriorate around the value with repeated inflation cycles or age. Since I very, very rarely experience flat tires anymore, I make it a point to change out tubes about every two years. My last tire blow out was in '86 so I think I can say tires have very good QC (I will pull off the road on a long descent on a hot day though to let rims cool). I did have a friend tell me a few years ago he ignored a concave rim and had it fail on him resulting in a broken wrist--but he said that was entirely his fault. Otherwise, with regular bike cleaning inspections, I think bicycles are amazingly dependable machines if one doesn't use inappropriate equipment and the main danger of cycling is operator error or automobile traffic.