First, in case you didn't know, the full scale Ultimate Bipe has flat tail surfaces.

Airfoiled tail surfaces are almost always fully sheeted and are usually heavier that solid flat sheet tails. They are a lot heavier than built up flat tails, which are usually not sheeted.

Airfoiled tail surfaces are a little smoother around center in high speed maneuvers (precision aerobatics)but they don't do a thing for slow speed stuff (3D).

Mounting them at the correct incidence can be a challange because the stab saddle is not flat and decent looking tail fairings can be a nightmare.

But as Tailskid said, they do look good on scale models of airplanes that had airfoiled tails. (Back to the first sentence above.)

Jim