These are one of the best alternatives that you can use for fillets. When a certain amount of force is applied to a structure, ribs help in distributing that force evenly over the entire surface. The thickness of the ribs plays an important part because they should not be too thick or thin otherwise they can break.
The surface finish created by FDM structures is rough as compared to the other processes. If you are creating curved or shallow-angled surfaces, then this rough finish is even more visible. In order to reduce this, you can adjust the angles.