I don't have statistics to back this up, but probably not significantly more or less common than simple fan failures. The issue is the consequences. When you have a water block on your CPU, the amount of thermal mass available to absorb heat is relatively tiny compared to a big block of copper and aluminum. If the water stops flowing for whatever reason, the CPU can reach critical temperatures way faster than if a CPU fan fails. Plus, if the water isn't flowing, is it because the pump failed, or did the water leave somewhere? If so, what did it get wet?
The vast majority of people running water cooling don't experience failure. I just don't like the risk / reward balance for a 24/7 server that's mostly unattended.