While I agree with you on the aspect of the open channel, the slot doesn't allow air to intrude like an open pipe above the water level in the sump, so a siphon can still be maintained and properly tuned, as well as stopped and re-started. But the trick is getting the flow needed over the screen to match the siphon, and these two numbers are not always perfectly matched. Balancing them out by adjusting the return pump flow and the gate valve can sometimes mean the siphon is "restricted" far back from it's full-open maximum flow, and this can result in a slow starting siphon, but it should purge and start eventually.
The backpressure issue can generally be solved by not allowing algae to grow into the slot/screen junction. I had this issue before on one of my first scrubbers, which was drain fed (not on a BA though) and the backpressure issue did cause the water to back up the standpipe and raise the tank level, but that was dues to slot/screen junction growth and likely could have been solved by blocking the light to that area...but I didn't know that at the time.
