I run a pf1200(with two utube) for about 14months and a pf1000(also two utube) for 12 months. Everything is good
I used to turn my return pump on and off twice a day while I feed my fish, that similar to a power outage, yet no failure. (I stopped doing that after I automated my system and move on to auto feeding)
Besides a couple of operating errors that can cause failure. These things are quite reliable.
1) air bubble getting into the utube. Either because the water level of your aquarium and your overflow box has a large enough difference and cause the water draining from your aquarium to smashed into the water in your overflow box and an air bubble is created and suck into the utube. Or your return is aim directly to the overflow box blowing water with air bubbles to it.
2) not enough flow, In most of the time air bubbles created by case, 1 will be flushed away when you have good flow coming from your return. But if your return flow rate is too low compare to your overflow box's flowrate rating. Those air bubbles will be trapped inside the utube and potentially break the siphon when your return pump stops.
Now couple things in mind, pf1200/pf1000 have 2 utube, in most cases, only one utube will break its siphon, the other will have a faster flow rate until it reaches its maximum capacity( i am not sure, you might have to do the math your self) and it will start flooding. And before it reaches its maximum capacity the water level in your tank will raise a bit, but it won't flood if you have enough room for those extra water.
For my own applications, I pulled one utube(pf1000) out and tested the results, it will raise my tank's water level roughly 0.5cm in a 120g 60' ×18' tank. That being said I am pumping 800+gph( actual gph' after all the head pressures and reduction, not a pump's rated gph)according to my apex flow meter.
Also, note when your tank's water level raises your sump's water level are lower if your sump doesn't have enough water at your return area/ water reserves area your return pump is going to be running without water and may cause damage to it.
Or you can drill a hole on the top of the tube and connect that to a maxi jet( or similar pump) to draw potential air bubbles out. ( i did for a few months, found it to be an unnecessary move)
Do I recommend an overflow box? Yes if you can't drill your tank like I am. But if you could drill your tank, drilling it is definitely a better way to go. It is cheaper, uses fewer spaces and more reliable.
