The 2 main concerns are that it provides an edge where snails/fish/algae/debris can get caught instead of passing smoothly through. This makes it the most likely clog point. The other concern is that you can throttle it closed so much that its inherent built in safety won't function. Ideally, when a drain like this starts to clog the water level rises until it goes full siphon. This dramatically increases water flow making overfilling the tank less likely. It sounds like a toilet flushing over and over again when this happens. Without enough excess drain capacity, even the full siphon may not work the way you want.
Thanks Im not using a siphon system just an over flow box. I have a strainer on it so snails wont get in.