I played with flaperons on a 2 meter glider a few years back. For small amounts of movement up to about 7 degrees they added lift with little or no bad side effects. The next little bit up to about 15 degrees started to show some tendency for adverse yaw dispite the fact I had a fair amount of differential in place. Beyond that they really started to have some bad side effects. I continued to use them as an airfoil camber changing tool but never used them at large movements for landings at all.

Later on an ARF with strip ailerons I tried flaperons again. I was sadly dissapointed at the amount of effect they had on the landing speed. Guess they just were not wide enough. But the adverse yaw showed up again and the problem you describe with a nasty stall showed up as well for any travel beyond about 10 degrees down. I never did try it with up reflexing.

The effect they have on models in general will depend a lot on the model layout. Conventional ailerons will benifit from some up reflex as only the outer portion of the wing will have the lift reducing reflex so it will appear that the wing now has some twist in it. However it may also depend on the airfoil as well. With the ailerons up a little you're now flying with a tip airfoil that is basically cambered upside down. Some airfoils may not take kindly to that and tip stall easier. Only an experimant will tell for sure. Do it at least 3 mistakes high at first.

On models with strip ailerons adding reflex probably won't help at all since you've added the reflex over the whole wing rather than just the tips.