The biggest issue is that if the u-tube loses its siphon you lose your drain. In this case your sump pump is filling the tank with nowhere for the water to go. I had issues with the one that came with my overflow box, it would lose siphon any time the pump was turned off. I bought one that was about an inch longer on each side and never had that issue again. Since a drilled tank doesn't rely on a siphon, just a drain, it takes a physical blockage to stop/reduce flow.
For any sump the best thing to do is to fill the tank until it is overflowing into the sump, then fill the sump all the way with the pump OFF. Then turn the pump on and mark on the sump where the water level drops down to. This is your safety fill level, the level at which the sump will not flood if the pump stops or the power goes out. This way you know how much you can top off with the pump on.