I asked Dr.
Mark Maugham about how much slip angle the winglets on my discus would tolerate before they stall. This was his response.

"In our flight tests, it wasn't possible to stall the winglet at any sideslips we could generate. There is a misconception that a winglet behaves as a vertical tail, but in reality, it operates in a "bubble" of induced velocities generated by the wing. So, when the glider is at some yaw angle, the flowfield around the winglet does not "experience" nearly as much."

I hope this helps.