Yes it can leak, and yes it could ruin your build. My first foray into watercooling (on the system I am on now) was the Cooler Master Eisberg 240L. Long story short, it leaked (on my new 980 GTX) after about 8 months. Without going into a long explanation, the seam on the plastic shell that housed the pump and copper plate started to separate. Fortunately I was able to get my system back up and running after a few days. It was touch and go though. I really thought I was going to have to send all my hardware into Cooler Master for them to test prior to them reimbursing me.
I do think it's a rare occurrence, but it does happen.
I was a little skeptical about using the replacement (Cooler Master Glacer 240L) that Cooler Master sent me, but I am happy to say that I did use it and it's been good thus far (just over a year).