Nearly every overflow sold on the market is relyable. Most of the time people have problems with them its because they fail to do simple preventive maintance like making sure the overflow is free of alage and debirs. Cleaning the overflow tube/channel, etc.
I personally prefer the U tube overflows over the C channel style overflow.
You can use a float switch mounted in the main display tank to cut the water to your sump pump. This will prevent a flood of the main tank. If water rises up and trigers the float switch you can set it to trigger a relay that opens the power circut on the pump. Thats about the only way I know of to guarintee you wont have a flood from the main tank overflowing. Of course you need to make sure your sump will have enough extra capacity to hold any water that might back syphon from the tank when the pump is off.
