The root bending issue has been well answered.
Ultralights and hang gliders often have tailless swept wings. Such a wing with an elliptical distribution can be unstable in pitch, especially if the aerofoil is cambered. The bell distribution effectively puts a "tail at the tips of the wings", making the plane stable.
The rule of thumb is that doubling the dimensions of an aircraft will give it four times the wing area and hence four times lifting capacity. It gives double the spar depth, which works out at twice the stress, so four times the bending moment. The spar is twice as long so that's also four times the weight, proportional to lift. Drag scales with frontal area plus wing area, i.e. also times four. So L/D is constant at all scales. Secondary effects such as materials usage efficiency and control authority provide challenges which the ingenious designer can often overcome.