Most of the world uses microlight definitions that include Vso<=35kCAS, that is clearly a very important design driver and tends to deliver low speed aircraft. The newest designs are achieving 80-90kCAS in level flight, which impresses microlight pilots, but in real terms is still very low speed.
And being low speed, form drag is a bit less problematic than it would on a higher speed aeroplane - but clearly not trivial, and all the cables and cantilevers create very draggy airframes. Billy did a great job with the PulsR but it's still, well, a flexwing!
There have been attempts to achieve much lower drag with a similar wingform in, say, the Dyke Delta or Verhees Delta. But they are using much more conventional structures, and don't use aeroelastic deformation to achieve stability in the way a Rogallo wing does.
G