The reliability of AIO coolers has come a long way. AIO leaks are exceedingly rare. Like any other mechanical device, there’s always the low possibility of a pump failure, but luckily, modern CPUs have protection built in should it get too hot. The bottom line is that you can run an AIO in your system with peace of mind. Coolers can 'degrade' over time as coolant slowly evaporates or falls out of the solution. Still, these kinds of things are becoming less of a concern as manufacturers learn new tricks and methods after years of testing and feedback. A modern AIO should serve you well for many years.