If all is as it should be (perfect world, lab/test bench scenario) they won't leak. Like anything else, there's always a chance. Probably less chance than a completely home built water cooling loop. Hard to tell what would be toast if it leaked. You may get lucky if you catch it in time. You may lose your mobo, you may lose your cpu, you may lose the whole works. It's a roll of the dice. Being that there is risk involved (more than air cooling) I'd check it every once in awhile. Look for visual signs of a leak, any discoloration, displacement of dust around the fittings things like that. If the hoses are bare rubber (no sheathing) give them a slight squeeze with your thumb and finger - see if they feel soft or are becoming stiff or brittle. Probably a good idea not to leave it running while you're away or overnight while you're sleeping. If something were to go wrong and you're there paying attention you stand a chance of reacting and shutting things down.
I don't see anything in the warranty to indicate they cover your system beyond replacement of the faulty cooler.