I have had, and on my 90 gallon cichlid tank still do, a HOB overflow. It has been running without failure for 2 years straight. Yes planning is important... plan for your sump to handle the water incase of failure, and your tank to handle the amount of water in your sump. I have read that some have placed there sump in a "tray" as a failsafe should the sump overflow. I have done two things that I feel are required when running a siphon (HOB) overflow.
1. auto top off to your sump... this prevents you from having too much water in your sump should there be a failure of the overflow.
2. Despite my overflow being a single U-tube design, I have a second U-tube running next to the original that is extra tall. This way... should there be a failure of one the long one NEVER will lose siphon because the ends are always below the minimum water level. Don't woory about too much water leaving the tank... the drain can only handle 600 gph and the supply to the tank is the same.
1. auto top off to your sump... this prevents you from having too much water in your sump should there be a failure of the overflow.
2. Despite my overflow being a single U-tube design, I have a second U-tube running next to the original that is extra tall. This way... should there be a failure of one the long one NEVER will lose siphon because the ends are always below the minimum water level. Don't woory about too much water leaving the tank... the drain can only handle 600 gph and the supply to the tank is the same.