I was also worried setting up my first overflow box. I think mine is a standard Eshopps with a rigid U/J tube. After setting it up. I ran the pump, shut it off, watched, ran again, stopped return pump, and watched the system until I felt comfortable with the set up. Nothing short of a space-time-gravity anomaly with break the siphon. No trouble with bubbles building up either.
A trick I discovered by accident, and one that will help get any excess air trapped in J-tube out. I was pouring top-off water straight into overflow box and it sucked the air bubble trapped in my J-tube. You will get the most flow with no air pockets. You could just scoop a pitcher of water from your tank and pour it in.
overflows can be very noisy if you don't have one of those sponge silencers. I had to fabricate one out of some filter pad, very quiet now, don't notice it at all, my rocks make more noise.
One issue I did encounter was finding the right return pump. My overflow box was used, but from looking online I was one of the smaller eshopps, rated at about 300 gph. My system is small. A 38gal with a 10 gal sump, almost nano really. I didn't need a large return. I started with small penguin power head rated at 175 I think, that was garbage and way too loud. never get penguin power heads unless you like lots of noise. Finally settled on a Maxi-Jet 1200 rated at 295gph. My head pressure on my return seemed minimal, even with a few 90 degree angles which are supposed to add a foot of pressure per angle, but I am still putting near the capacity of the overflow. The return itself gives a lot of flow for a tiny 38gal.
My advice is if you need piece of mind, build a fail safe of some kind, but if its all set up right shouldn't need to worry about it. physics will do its thing. and it will free up precious power sockets!
That has been my experience thus far on overflow boxes.
