The only problem ive had was with a ready-to-go watercooling package i bought when i first tried out watercooling. It was filled with water and connected prior to delivery. Unfortunatly there was a leak where the tube connected to the CPU block, causing water to drip onto my gfx card and down onto the CPU socket.
Fortunately both survived(though the gfx card had to dry off for a week
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After that i made my own watercooling system, and too avoid problems like these, i made a piece of tube that stuck up into the air, so that the water could expand and contract without causing any pressure changes in the system. After that ive never had any problems at all.