Peltier modules themselves are very reliable since they are solid-state devices. They will start to overheat if you put to much voltage across them, and if condensation or moisture gets inside the peltier them it will corrode and die. Also the external plates of the pelt are subjected to heating and cooling when you turn on the PC and turn it off again, which could stress the peltier. I aven't heard of anyone killing the module this way but it is something to think about.
The more important aspect of reliability is the condensation-proofing of your CPU, motherboard and if you are running a waterchiller the pipes and waterblock. Neoprene for foam insulation, dielectric grease in and around the socket and a load of conformal coating around the back of the motherboard, possibly surrounding the socket on the front too.
