I've used a dual float switch system from autotopoff.com for the past 6 to 7 months.
I have not had a single flood due to the float switch's malfunctioning. I did have one flood because a too large of a snail wanted to take an adventure in my J tube overflow. It blocked the flow,so the return pump pumped my DT full of water while the autotop off kicked on to fill the sump as it emptied. THANK GOD I was home and watching the tank when it happened! (Pure coincidence! I normally would have been in bed at that time!)
So, I took matters to control what I could. I put additional neeting over my overflow box to prevent snails from getting curious. I put my auto top off switch on a mechanical timer. (It's only in 15 min increments, would like to get into 1 min or less increments).
This way, even if something were to happen, the top off would only kick on 2 or 3 times a day, whenever I set it, and those times are when I most likely would be home viewing the tanks.
Since taking these precautions. I have not had a single issue with overflows. I've seen snails investigating the overflow box, but not a one has made it inside. so, between the dual float switches and the timer, things have been good!