Couple of things 'wrong' with wood screws in balsa, or any of the light hobby woods for that matter.

First, wood screws are tapered, and get their holding strength from deforming the wood where the taper is thicker than the pilot hole; depending on the pilot drill used, the wood can be deformed all along the length of the screw. The wood doesn't deform in one plane, i.e. away from the screw, it deforms in every direction which means that as the screw is run in the surface of the wood bulges (a very small amount, but it does bulge) which in turm may mean that the item being fastened does not make flat contact with the wood surface but rather comes to rest on a 'mound', and therein is not as stable as it would be if on a truly flat surface.

Second thing 'wrong' with wood screws in balsa is that balsa, and spruce to a lesser extent, has it's principal strengths in tension and compression, but is quite low in resistance to bending, and is abysmal in resistance to tear-out. That's why you have to harden the wood before installing treaded fasteners of any sort - they simply won't hold otherwise.

Control horns intended for attachment to balsa flight control surfaces nearly always have a backing plate, and is attached with machine screws where the screws go through the horn, the flight control surface, and the backing plate, and is locked into the backing plate. Like [link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXK107&P=ML]this.[/link]

This type of control horn works on tapered flight control surfaces as well as those where the upper and lower surfaces are parallel.

Don't worry about simple building errors because you cannot possibly have made all the mistakes yet; we're still inventing dumb ways to do things.