Sumps/overflows can fail. For instance if the sump is too full, then in the event of a power failure the sump itself will overflow. So you need to design sumps very carefully. They need to hold enough water to resist underflow due to evaporation for a reasonable length of time, but not so much that they can't hold the residual overflow from the tank when the power goes off. And there needs to be enough room in the tank that if the overflow fails, the additional water pumped up from the sump does not flood it.
The thing I notice about my new 75 gallon is that the return line has a hole on the sump side above water level that drips water into the overflow prevents back siphonings, so when the power to sump is cut the only water that fill up my sump is the water in the pump line which probably less that a gallon, I stupidly went back to lfs
saying it was broke, when they told me what it was it made so much sense, the lfs
good but only accurate about 90% time(tried to sell me a "reef safe starfish" that was really a questionable starfish)