With very slow speed flight, the flow is laminar. Anything much below a Reynolds number of 20k. Above that, to the upper hundred thousands, the airflow can be turbulent, have hysteresis effects, seperation bubbles.
And indoor planes really don't care about drag.. look at all the bracing strands on the really light microfilm planes.
A "single surface wing" is better at that slow flight.
Outdoors, drag does become important, and the simple single surface can't perform well.
When properly used, most anything works. When it's misapplied, there will be better choices.