Overflow boxes are very reliable when used properly. The U-tube style is the preferred style, with at least one company (LifeReef) guaranteeing that their overflow boxes will not fail / lose siphon from a power or pump failure. We've been running a U-tube style overflow box for over 3 years now without a single failure on our 90g, and know of one running at a LFS for over 10 years without a single failure.
Having water back-siphon down the return line is a far greater concern during a power outage and a much more common reason that people have when they flood their sump / floor, at least IME and from the people that I know that have had such issues. It's also very easily prevented.
