I go with U tube overflows explicitly because in power outages they do NOT lose syphon. Drilling small holes just below the water level on your return lines will break the backsyphon of the water draining back into the sump fro your return lines. Making sure the return lines are not to deep in the water and having a sump of enough capcity to hold the water that does backflow is another way to help protect yourself from floods.
One way flap valves are also of a benifit to protect from the back syphon. All of these methods combined will help even more.
Syphons break when air is introduced so yes having holes drilled in your return lines will break the syphon assuming the holes are kept free and clear of algae and are not allowed to get covered with corline. The coralien will effectivly seal the holes thus negating their benifit.
It all comes down to regular upkeep. Check your overflow daily for things that might be obstruting the drain lines. Check the sump level and never fill it beyond an established "full" line. This line should be established by testing the power outage situation and making sure that there is enough reserve capacity in the sump to hold any backflow from your returns. If you have the syphon break holes make sure they are free and clear. An overflow is not "Set it and forget it".
Sorry but I had to toss that line in as its engraned in my brain from that stuipd infomercial.