Flat profiles with a thickness/chord ratio of less than 9 % are good for a high sailing speed. For gaining a good lift, they also need high speed. Therefore, their initial production of lift is not that good. For maximum lift they need a rather small angle of attack. The practical consequence for the sailor is that the upwind speed depends to a large extent on the sailing speed and not so much on the angle of attack.
On the other hand a full profile will allow a bigger angle of attack in combination with reduced speed. Thicker fins do not accelerate well but they give better control and manoeuvrability. On tracks across the wind or downwind they are slower and do not give a lively feeling.