I couldn't harrier the UCD last year but after practicing with a SPAD, some foamies, and a Mini 3D (which is a no-brainer in harrier) the UCD now magically harriers

I use 50-70% spoileron. It's all about finding the sweet spot and keeping it there. Slowly burry the elevator (45-60deg) while adding a few clicks of throttle. The prop wash really helps get the plane through the stall.

Or do a parachute. Nose down at idle, pop in full elevator, flick on the spoilerons, add a few clicks of throttle to stabilize the plane in a nice elevator, a few more clicks and you have the harrier.

Not the easiest plane to harrier, you can get "out of shape" quick.