Theoretically, the higher you place the "T" the more pressure you will need to restart the overflow. If there is not enough pressure, the rather might fill the display before the overflow catches up. (not positive on this. One, but I will. Ask)
If built properly, the chance of something failing are slim. Thou, you are right, there is always a chance, however small it may be. This is why drilling the tank is really the only failsafe method. Though even then, if your overflow bulkhead clogs, or the piping clogs, you still run that slight risk. I guess its just a matt what you're comfortable with. I know people who have been running their PVC overflows for many years with no problems, and I have yet to run into any problems after initial set up. But other people may not be willing to chance it.