And even then..you would want some kind of manual override or bypass so that you can still have water on (when you want it) if power does go out.
As hj says, there are trade-offs between any of the systems. Either they won't work when needed (faulty sensor, no power, not enough flow to trigger, etc.), will work too late (leak in an area where you don't have the sensors), or will cause nuisance trips with regular use (causing you to disarm the thing because you are tired of resetting it all the time).
With enough complexity, a system could a devised that was 'fail-safe', but it would be complex, expensive, and you would need sensors all over the place. Remember, that a leak may not only develop at a sink, tub, etc., but could be anywhere in the house where plumbing is ran. You could have a connector fail, pinholes develop, a screw that was put into the copper pipe by the drywall crew finally rust enough to leak, etc.
If you worry about it, just make sure that you have good insurance that will cover everything you need it to (including dealing with mold) and just don't worry about it. The truth is that even if you perfectly protected everything from piping leaks, you could still flood and these devices won't help you (sewer backup, heavy rains, failed sump pump/ground water, etc.).