Here's why we try all the other solutions first.

You can't put mass balances inside a model wing - the control surface deflections are too great to keep the balance weights completely within the wing. Mass balances at the end of the ailerons don't work because light weight ailerons cannot be made stiff enough in torsion. Leading edge mass balances are impractical in models - in full size they use depleted uranium and Frieze ailerons both of which help a lot. That only leaves external mass balances - weights on arms extending forward of the aileron hinge line and normally mounted on the bottom so they are not such an eyesore.

Disadvantages;
1. Increases live weight and moment of inertia of control system.
2. Ugly.
3. Added complexity.
4. When the wheels fold (belly landing) the mass balances are torn off and the servo gears are stripped. Even a wing drop on landing can result in this if the grass is long.
5. Difficult to fit wing bags.
6. Target for handling and transport damage.
7. Additional point of failure - if the weight falls of in flight you have a problem.

Light, stiff, friction and slop free - that's what it takes.